When inserting code {android:theme="@android:style/Theme.NoTitleBar"} in AndroidManifest.xml, Unfortunately my app has stopped

my Manifest runs like this;

 <uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="21" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" 

    >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name"   
        android:theme="@android:style/Theme.NoTitleBar"    
         >


        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>



</application>

    <uses-permission android:name="android.permission.INTERNET"/>

---

But deleting android:theme="@android:style/Theme.NoTitleBar" in activity, it works. It's been about 1 month struggling Eclipse. Any advice will be welcomed. From Korea