Make sure you have specified the default activity in your AndroidManisfest.xml file. Within your default activity‘s xml<activity> section, there should be these tags:
<intent-filter> <action android:name="android.intent.action.MAIN"/> <category android:name="android.intent.category.LAUNCHER"/> </intent-filter>
即
<activity android:name=".MainActivity"> <intent-filter> <action android:name="android.intent.action.MAIN"/> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity>
这样就会把 MainActivity 这个Activity设置为默认。
Error in Android Studio - "Default Activity Not Found"
原文：http://www.cnblogs.com/longzhongren/p/6068754.html