Deficiencies in conventional frequency dispersion models of GaAs MESFET are addressed regarding their large-signal response, A new model which can accurately describes the large-signal dynamic properties is proposed, Only de and scattering parameter data are required to extract the model parameters, which can be easily implemented into microwave circuit simulators in the macro circuit form, The validity of this model is demonstrated by comparing predicted pulsed I-V characteristics and power saturation characteristics with measured ones.