Abstract: The influence of the frequency on the thermal burnout effect of a positive-intrinsic-negative(PIN) diode limiter is studied with numerical simulation and theoretical analysis. The study shows that the microwave pulse thermal burnout power threshold of a PIN diode limiter decreases with the increase of the frequency under the same microwave pulse width. This is due to the fact that although the frequency is irrelevant to the microwave pulse energy under the same power, the number of the periodic increases of the PIN diode impedance rises with the increase of the frequency under the same pulse width. Thus, the average impedance and the actual absorbed energy of the PIN diode increase, too. This phenomenon is significant for the comprehensive evaluation of the protective capability of a PIN diode limiter to microwave pulses.