An effect of source harmonic tuning on linearity of power GaAs FET's under class AB operation is demonstrated. To improve efficiency of the power amplifiers, GaAs FET's are often poerated under class AB condition. Due to lower bias current, a class AB amplifier begins to show nonlinearity at lower input power comparing with class A operation, and as the power level of the input signal increases, however, an output power sometimes increases abruptly. From nonlinear circuit simulation, we have found this phenomenon is occurred by the distortion in gate RF voltage, and by suppressing even-order harmonics in the input circuit of GaAs FET, class AB amplifiers can be effectively linearized. In this paper, we show the condition for improving the linearity of power CaAs FET's under class AB operation by the source harmonic tuning technique.