never mind...
the problem was that I also copyed empty RTF to the data object...
 I have a copy code as below. Which works well inside my application.
 But if I try to copy a picture into word, it always fail!
 Any tips?

 =====
 public static void SetPasteboard(Object obj)
 {
     IDataObject ido = new DataObject();
     ido.SetData(obj);

     if (obj is Bitmap)
         ido.SetData(DataFormats.Bitmap, obj);

     Clipboard.SetDataObject(ido);
 }