<Button android:background="#e0000000" ... /> 

<Button android:background="#00000000" ... /> 

//Java代码 
View v = findViewById(R.id.content);//找到你要设透明背景的layout 的id 
v.getBackground().setAlpha(100);//0~255透明度值 ，值越小越透明 

<Button android:background="@android:color/transparent"/> 

<color name="translucent">#55000000</color> 

<style name="myDialog" parent="@android:style/Theme.Dialog"> 
  <item name="android:windowFrame">@null</item> 
  <item name="android:windowIsFloating">true</item> 
  <item name="android:windowIsTranslucent">true</item> 
  <item name="android:windowNoTitle">true</item> 
  <item name="android:windowBackground">@color/translucent</item> 
  <item name="android:backgroundDimEnabled">false</item> 
  <item name="android:windowCloseOnTouchOutside">false</item> 
</style> 

<resources> 
 <style name="Transparent"> 
 <item name="android:windowBackground"> 
  @color/transparent_background 
 </item> 
 <item name="android:windowNoTitle">true</item> 
 <item name="android:windowIsTranslucent">true</item>  
 <item name="android:windowAnimationStyle"> 
   @+android:style/Animation.Translucent 
 </item> 
 </style> 
</resources> 

<?xml version="1.0" encoding="utf-8"?> 
<resources> 
 <color name="transparent_background">#50000000</color> 
</resources> 

<activity 
android:name=".TransparentActivity" 
android:theme="@style/Transparent"> 
</activity> 

public void onCreate(Bundle savedInstanceState) { 
  super.onCreate(savedInstanceState); 
  setTheme(R.style.Transparent);  
  setContentView(R.layout.transparent); 
  // ... ... 
}