Abstract: A time-domain model of PIN diode limiter has been studied with PSpice code in this paper. The Q-I relationship of the I-region is obtained using the Laplace transform and Padé approximation based on the base area ambipolar carrier transport equation. The proposed limiter circuit model takes into account the input RF power, the pulse rising time and the width of the I-region. Simulation results and experimental studies show that PIN diode limiter spike leakage characters are determined on the microwave pulse power and rising time. It is a transient effect and might not be observed with a slow rising time pulse.