Are 29" and 26" bikes come with different size frames? I am going to buy a cycle. In this regard I got my framesize 20". Now I would like to know is it for 26er or 29er ?? Is frame size will be different for 29er and 26er ??

I am going to buy a GT Timberline. My height is 5' 11.5" inseam is 34.5". I am from Bangladesh. May be I am going to have 18.5" frame size.
When 29ers first became popular many buyers found that many manufacturers did not adjust their size charts to allow for the increased standover height. Whether this is the case for the models you are looking at is impossible to tell without more information. With identical main frame geometry, a 29er will be taller than its' 26" counterpart. You really have to test ride or test fit each to determine which size is correct for you. If you are comparing sizes between different brands there may also be a size differences. Brand "A" may require a size 20", Brand "B" may require a different size.