diff --git a/lib/cmd/cov.js b/lib/cmd/cov.js
index b6c4404d..2dc2b9b1 100644
--- a/lib/cmd/cov.js
+++ b/lib/cmd/cov.js
@@ -42,7 +42,6 @@ class CovCommand extends Command {
 
   * run(context) {
     const { cwd, argv, execArgv, env } = context;
-
     if (argv.prerequire) {
       env.EGG_BIN_PREREQUIRE = 'true';
     }
@@ -73,7 +72,10 @@ class CovCommand extends Command {
     const opt = {
       cwd,
       execArgv,
-      env: Object.assign({ NODE_ENV: 'test' }, env),
+      env: Object.assign({
+        NODE_ENV: 'test',
+        EGG_TYPESCRIPT: context.argv.typescript,
+      }, env),
     };
 
     // save coverage-xxxx.json to $PWD/coverage
diff --git a/lib/command.js b/lib/command.js
index 63f2ed1f..544e32b7 100644
--- a/lib/command.js
+++ b/lib/command.js
@@ -19,6 +19,7 @@ class Command extends BaseCommand {
         description: 'whether enable typescript support, will load `ts-node/register` etc',
         type: 'boolean',
         alias: 'ts',
+        default: undefined,
       },
     };
   }
@@ -33,14 +34,16 @@ class Command extends BaseCommand {
     // remove unuse args
     argv.$0 = undefined;
 
-    // read `egg.typescript` from package.json
-    let baseDir = argv._[0] || argv.baseDir || cwd;
-    if (!path.isAbsolute(baseDir)) baseDir = path.join(cwd, baseDir);
-    const pkgFile = path.join(baseDir, 'package.json');
-    if (fs.existsSync(pkgFile)) {
-      const pkgInfo = require(pkgFile);
-      if (pkgInfo && pkgInfo.egg && pkgInfo.egg.typescript) {
-        argv.typescript = true;
+    // read `egg.typescript` from package.json if not pass argv
+    if (argv.typescript === undefined) {
+      let baseDir = argv._[0] || argv.baseDir || cwd;
+      if (!path.isAbsolute(baseDir)) baseDir = path.join(cwd, baseDir);
+      const pkgFile = path.join(baseDir, 'package.json');
+      if (fs.existsSync(pkgFile)) {
+        const pkgInfo = require(pkgFile);
+        if (pkgInfo && pkgInfo.egg && pkgInfo.egg.typescript === true) {
+          argv.typescript = true;
+        }
       }
     }
 
diff --git a/test/fixtures/example-ts-pkg/agent.js b/test/fixtures/example-ts-pkg/agent.js
new file mode 100644
index 00000000..2fbe626b
--- /dev/null
+++ b/test/fixtures/example-ts-pkg/agent.js
@@ -0,0 +1,6 @@
+'use strict';
+
+
+module.exports = agent => {
+  console.log(`agent.options.typescript = ${agent.options.typescript}`);
+};
diff --git a/test/fixtures/example-ts-pkg/test/index.test.ts b/test/fixtures/example-ts-pkg/test/index.test.ts
index 25c83639..d4599f69 100644
--- a/test/fixtures/example-ts-pkg/test/index.test.ts
+++ b/test/fixtures/example-ts-pkg/test/index.test.ts
@@ -2,7 +2,6 @@
 
 import { Application, Context } from 'egg';
 import { default as mock, MockOption, BaseMockApplication } from 'egg-mock';
-import * as path from 'path';
 
 describe('test/index.test.ts', () => {
   let app: BaseMockApplication<Application, Context>;
diff --git a/test/ts.test.js b/test/ts.test.js
index 0938b4a3..bff49e3d 100644
--- a/test/ts.test.js
+++ b/test/ts.test.js
@@ -90,6 +90,15 @@ describe('test/ts.test.js', () => {
         .end();
     });
 
+    it('should fail start app with --no-ts', () => {
+      return coffee.fork(eggBin, [ 'dev', '--no-ts' ], { cwd })
+        // .debug()
+        .expect('stdout', /agent.options.typescript = false/)
+        .expect('stdout', /started/)
+        .expect('code', 0)
+        .end();
+    });
+
     it('should start app with relative path', () => {
       return coffee.fork(eggBin, [ 'dev', './example-ts-pkg' ], { cwd: path.dirname(cwd) })
         // .debug()