When I test it on emulator, no problem. But when I test it on real device (iPad), this problem occurs. There are several images with alpha which stake on each other I suspect that could be the problem but I would like to know anyone who had this problem before. I will try and see if I could fix this. Thanks.
Problem solved. It is a bug of iPad2, see: http://www.cocos2d-x.org/news/24
I just add in
and problem solved. however this might affect some transition effects which rely on 3d projection.