Abstract: A certain high-voltage circuit breaker operated abnormally after 300 opening and closing tests, and upon disassembly, it was found that the first gear shaft had broken teeth. The reasons for gear shaft tooth breakage were analyzed using macroscopic observation, chemical composition analysis, hardness and carburizing layer depth testing, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that during carburizing, the temperature and carbon potential were too high, resulting in the formation of coarse needle like martensite and network carbides in the microstructure, which increased the brittleness of the material. During the rotation of the gear shaft, microcracks first appear near the brittle phase at the tooth root. Under stress, the cracks further propagate along the grain boundaries, ultimately led to intergranular fracture of the gear shaft.