B738 Flap
 

You could have a miss rigged flap. We had a 737-700 come right out of Boeing like this. As mentioned youn could also have an Autopilot problem.
You couls swao A and B for T/S but as was also mentioned you need a little more info before diagnosing this one.

I know why. Since it goes to the left every time you put 40 degrees of flap, it means that the flap on the right wing is slightly slower to respond then the flap on the left wing. Although the pilot could react exactly the way he reacts to a crosswind, its a major problem. Just like the left main gear on the A-380. It is 6 or 7 secs slower to react than the right most one... MAJOR disadvantage. If on an emergency landing you forget to put the gear until the last second, well lets say the people on the left die first...

We have 10 738 in our fleet and and problem same with all.The slight left bank occur on autopilot A and B either manual flight when the put 40 flap.I dont have classic 737 experience and this is my second jet after MD83.I heard
both wing dihedral angle are not same.Very small different between both dihedral angle and it makes small bank when use of 40 flap.Did u hear anything abaout it before?

We used to have an aircraft (not a 737) that did this, and it was "always like that" with nobody curious enough to determine why, as it was easily controllable. Then a newhire, doing his first preflight on an actual aircraft, discovered that the 50-70 odd vortex generators present on the right inboard flap did not appear to exist on the left flap. This D-model aircraft had been flying around with a C-model flap for years. Some strong-willed mechanic got it to fit, and that was that.