Abstract: In the study of anti-HPM reinforcement of analog integrated-circuits, the damage mechanism of a single BJT in the tested ICs is researched first. The research is composed of effect experiments and failure analysis, in order to get the damage mechanism of BJTs. In the experiments, microwave energy is injected into the BJT from the base, the emitter and the collector respectively, till the transistors are damaged. The damage threshold, degradation and failure of tested transistors are observed. Failure analysis of damaged transistors compare the damaged position and the failure mode of damaged transistors, showing that transistors have the maximum vulnerability to microwave injected from the base, and the minimum vulnerability to microwave from the collector.