Small-signal high frequency characteristics of 4H-SiC MESFET has been studied by using two-dimensional numerical drift-diffusion model in frequency domain, Non-ideal Schottky boundary conditions have been introduced that take into account a thin interfacial layer and interface energy states. It has been demonstrated that the 10 dB/dec small signal current gain roll-off can be attributed to the existence of high density interface states at the metal-semiconductor interface. It has been found that as the gate length is reduced to 0.1 mu m, f(T) and f(max) may reach as high as 30 and 62 GHz, respectively.