A method for fabricating 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 gate dielectric layer on the barrier layer; forming a work function metal layer on the gate dielectric layer; patterning the work function metal layer and the gate dielectric layer; forming a gate electrode on the work function metal layer; and forming a source electrode and a drain electrode adjacent to two sides of the gate electrode.