I wonder if any one could tell me ,why does the current gain change as the frequency of ac supply changes?

advanced thanks.

 

A transistor uses a DC supply, not an AC supply.

Maybe you are talking about the signal frequency. The datasheet for the 2N3904 transistor by Fairchild shows a graph of the AC current gain (hfe) dropping at high frequencies until the gain is zero at the transition frequency.

The reduced gain at high frequencies is caused by internal and stray capacitance.