Abstract: A modified small signal equivalent circuit with gate voltage-varied conductance for SiC MESFET is proposed in this paper,and the fitted S11 is improved remarkably compared with conventional GaAs MESFET small signal equivalent circuit.A simple and accurate method based on full RF characteristic to directly determine the small-signal equivalent circuit of SiC MESFET is also proposed and validated.The model and method have been implemented in civil SiC MESFET processing line,and the determined equivalent circuit fits the S-parameters very well up to 18GHz.