"TW, I have init the port3 as 00111101b, my program doesn't work, if i set port3 as 11111111b, it work..."

look at the three 80C51_FAM_*.pdf documents. They can be downloaded from:
http://www.semiconductors.philips.com/products/...other.html

They will show you what the effect of p3.6 and p3.7 low is and why it does not work when they are low

Have fun,

Erik