A large-signal model for GaAs FET is derived based on its small-signal S parmeter and DC characteristics. The harmonic balance algorithm is applied to analyze and optimize the FET fundamental and harmonic oscillators, and the values of steady current are obtained. In the solving process, a simplified CAD approach is used to obtained the parameters of matching network when the output power is maximum. Finally, a fundamental oscillator and a harmonic oscillator of Q-band are fabricated. The measurements show that the theoratical
analysis and experimental results are in good agreement.