diff --git a/lib/egg.js b/lib/egg.js
index a32fa55a..8a2c2f6b 100644
--- a/lib/egg.js
+++ b/lib/egg.js
@@ -4,6 +4,7 @@ const assert = require('assert');
 const fs = require('fs');
 const KoaApplication = require('koa');
 const EggConsoleLogger = require('egg-logger').EggConsoleLogger;
+const debug = require('debug')('egg-core');
 
 const DEPRECATE = Symbol('EggCore#deprecate');
 
@@ -165,6 +166,8 @@ class EggCore extends KoaApplication {
     }).on('ready_timeout', id => {
       this.console.warn('[egg:core:ready_timeout] 10 seconds later %s was still unable to finish.', id);
     });
+
+    this.ready(() => debug('egg emit ready, application started'));
   }
 
 }
diff --git a/lib/loader/mixin/custom.js b/lib/loader/mixin/custom.js
index e4d70b1a..ab64ba7b 100644
--- a/lib/loader/mixin/custom.js
+++ b/lib/loader/mixin/custom.js
@@ -2,6 +2,8 @@
 
 const path = require('path');
 
+const LOAD_CUSTOM_FILE = Symbol('EggCore#loadCustomFile');
+
 module.exports = {
 
   /**
@@ -18,19 +20,50 @@ module.exports = {
    *   doAsync(done);
    * }
    * ```
+   *
+   * it's better to use async function
+   *
+   * ```js
+   * module.exports = async function(app) {
+   *   await doAsync();
+   * }
+   * ```
+   *
+   * Or use generator wrapped by co.wrap
+   *
+   * ```js
+   * const co = require('co');
+   * module.exports = co.wrap(function*(app) {
+   *   yield doAsync();
+   * });
+   * ```
    * @since 1.0.0
    */
   loadCustomApp() {
-    this.getLoadUnits()
-      .forEach(unit => this.loadFile(path.join(unit.path, 'app.js')));
+    this[LOAD_CUSTOM_FILE]('app.js');
   },
 
   /**
    * Load agent.js, same as {@link EggLoader#loadCustomApp}
    */
   loadCustomAgent() {
-    this.getLoadUnits()
-      .forEach(unit => this.loadFile(path.join(unit.path, 'agent.js')));
+    this[LOAD_CUSTOM_FILE]('agent.js');
   },
 
+  [LOAD_CUSTOM_FILE](filename) {
+    this.getLoadUnits()
+      .forEach(unit => {
+        const filepath = path.join(unit.path, filename);
+        const ret = this.loadFile(filepath);
+        registerCallback(ret, this.app, filepath);
+      });
+  },
 };
+
+function registerCallback(ret, app, filepath) {
+  // register readyCallback if custom file export async function
+  if (ret instanceof Promise) {
+    const done = app.readyCallback(filepath);
+    ret.then(() => done()).catch(done);
+  }
+}
diff --git a/package.json b/package.json
index dff6810b..f6066daa 100644
--- a/package.json
+++ b/package.json
@@ -32,6 +32,7 @@
   },
   "devDependencies": {
     "autod": "^2.7.0",
+    "co": "^4.6.0",
     "egg-bin": "1",
     "egg-ci": "1",
     "eslint": "~3.1.0",
diff --git a/test/fixtures/custom-app-async-function/app.js b/test/fixtures/custom-app-async-function/app.js
new file mode 100644
index 00000000..0c814d7c
--- /dev/null
+++ b/test/fixtures/custom-app-async-function/app.js
@@ -0,0 +1,20 @@
+"use strict";
+var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+    return new (P || (P = Promise))(function (resolve, reject) {
+        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
+        function rejected(value) { try { step(generator.throw(value)); } catch (e) { reject(e); } }
+        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
+        step((generator = generator.apply(thisArg, _arguments)).next());
+    });
+};
+function default_1(app) {
+    return __awaiter(this, void 0, void 0, function* () {
+        yield wait(1000);
+        app.app = true;
+    });
+}
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.default = default_1;
+function wait(timeout) {
+    return new Promise(resolve => setTimeout(resolve, timeout));
+}
diff --git a/test/fixtures/custom-app-async-function/app.ts b/test/fixtures/custom-app-async-function/app.ts
new file mode 100644
index 00000000..4eaec8dc
--- /dev/null
+++ b/test/fixtures/custom-app-async-function/app.ts
@@ -0,0 +1,8 @@
+export default async function(app) {
+  await wait(1000);
+  app.app = true;
+}
+
+function wait(timeout) {
+  return new Promise(resolve => setTimeout(resolve, timeout));
+}
diff --git a/test/fixtures/custom-app-async-function/package.json b/test/fixtures/custom-app-async-function/package.json
new file mode 100644
index 00000000..3d3a03f9
--- /dev/null
+++ b/test/fixtures/custom-app-async-function/package.json
@@ -0,0 +1,3 @@
+{
+  "name": "custom-app-async-function"
+}
diff --git a/test/fixtures/custom-app-error/app.js b/test/fixtures/custom-app-error/app.js
new file mode 100644
index 00000000..e6c6d580
--- /dev/null
+++ b/test/fixtures/custom-app-error/app.js
@@ -0,0 +1,12 @@
+'use strict';
+
+const co = require('co');
+
+module.exports = co.wrap(function*(app) {
+  yield wait(1000);
+  throw new Error('load async error');
+});
+
+function wait(timeout) {
+  return new Promise(resolve => setTimeout(resolve, timeout));
+}
diff --git a/test/fixtures/custom-app-error/package.json b/test/fixtures/custom-app-error/package.json
new file mode 100644
index 00000000..858197f6
--- /dev/null
+++ b/test/fixtures/custom-app-error/package.json
@@ -0,0 +1,3 @@
+{
+  "name": "custom-app-error"
+}
diff --git a/test/fixtures/custom-app-promise/app.js b/test/fixtures/custom-app-promise/app.js
new file mode 100644
index 00000000..9a80b6cb
--- /dev/null
+++ b/test/fixtures/custom-app-promise/app.js
@@ -0,0 +1,12 @@
+'use strict';
+
+const co = require('co');
+
+module.exports = co.wrap(function*(app) {
+  yield wait(1000);
+  app.app = true;
+});
+
+function wait(timeout) {
+  return new Promise(resolve => setTimeout(resolve, timeout));
+}
diff --git a/test/fixtures/custom-app-promise/package.json b/test/fixtures/custom-app-promise/package.json
new file mode 100644
index 00000000..32abf9cb
--- /dev/null
+++ b/test/fixtures/custom-app-promise/package.json
@@ -0,0 +1,3 @@
+{
+  "name": "custom-app-promise"
+}
diff --git a/test/loader/mixin/load_custom_app.test.js b/test/loader/mixin/load_custom_app.test.js
index c334a879..536b0f60 100644
--- a/test/loader/mixin/load_custom_app.test.js
+++ b/test/loader/mixin/load_custom_app.test.js
@@ -3,29 +3,83 @@
 const should = require('should');
 const utils = require('../../utils');
 
-describe('test/loader/mixin/load_custom_app.test.js', function() {
-
-  let app;
-  before(function() {
-    app = utils.createApp('plugin');
-    app.loader.loadPlugin();
-    app.loader.loadConfig();
-    app.loader.loadCustomApp();
+describe('test/loader/mixin/load_custom_app.test.js', () => {
+
+  describe('app.js as function', () => {
+    let app;
+    before(() => {
+      app = utils.createApp('plugin');
+      app.loader.loadPlugin();
+      app.loader.loadConfig();
+      app.loader.loadCustomApp();
+    });
+    after(() => app.close());
+
+    it('should load app.js', () => {
+      app.b.should.equal('plugin b');
+      app.c.should.equal('plugin c');
+      app.app.should.equal('app');
+    });
+
+    it('should app.js of plugin before application\'s', () => {
+      (app.dateB <= app.date).should.equal(true);
+      (app.dateC <= app.date).should.equal(true);
+    });
+
+    it('should not load plugin that is disabled', () => {
+      should.not.exists(app.a);
+    });
   });
-  after(() => app.close());
 
-  it('should load app.js', function() {
-    app.b.should.equal('plugin b');
-    app.c.should.equal('plugin c');
-    app.app.should.equal('app');
+  describe('app.js as function return promise', () => {
+    let app;
+    before(done => {
+      app = utils.createApp('custom-app-promise');
+      app.loader.loadPlugin();
+      app.loader.loadConfig();
+      app.loader.loadCustomApp();
+      app.ready(done);
+    });
+    after(() => app.close());
+
+    it('should load app.js success', () => {
+      app.app.should.be.true();
+    });
   });
 
-  it('should app.js of plugin before application\'s', function() {
-    (app.dateB <= app.date).should.equal(true);
-    (app.dateC <= app.date).should.equal(true);
+  describe('app.js as async function', () => {
+    let app;
+    before(done => {
+      app = utils.createApp('custom-app-async-function');
+      app.loader.loadPlugin();
+      app.loader.loadConfig();
+      app.loader.loadCustomApp();
+      app.ready(done);
+    });
+    after(() => app.close());
+
+    it('should load app.js success', () => {
+      app.app.should.be.true();
+    });
   });
 
-  it('should not load plugin that is disabled', function() {
-    should.not.exists(app.a);
+
+  describe('app.js load async error', () => {
+    let app;
+    after(() => app.close());
+
+    it('should load app.js success', done => {
+      app = utils.createApp('custom-app-error');
+      app.on('error', err => {
+        err.message.should.eql('load async error');
+        done();
+      });
+      app.loader.loadPlugin();
+      app.loader.loadConfig();
+      app.loader.loadCustomApp();
+      app.ready(() => {
+        throw new Error('should not call');
+      });
+    });
   });
 });