A method for fabricating a high electron mobility transistor (HEMT) includes the steps of forming a buffer layer on a substrate, forming a barrier layer on the buffer layer, forming a p-type semiconductor layer on the barrier layer, forming a gate electrode on the p-type semiconductor layer, and then forming a source electrode and a drain electrode adjacent to two sides of the gate electrode. Preferably, the buffer layer further includes a bottom portion having a first carbon concentration and a top portion having a second carbon concentration, in which the second carbon concentration is less than the first carbon concentration and a thickness of the bottom portion is less than a thickness of the top portion.