Abstract: By using the finite-difference time-domain method in two-dimension, the transient response of BJT (bipolar junction transistor) under the high power electromagnetic pulse was simulated. The distribution and variation of the electronic field, current density, and the temperature in the transistor during the burnout of the device were obtained and analyzed. The formations of hot spots during the process under the pulse with low and high magnitude were observed. That how the time and energy needed for burnout depended on the magnitude of the pulse was also obtained. When the magnitude of the pulse is relatively low, the hot spot locates near the collector in the breakdown tunnel. When the magnitude of the pulse is sufficiently high, the voltage between the base and the emitter will become ver