Abstract: The connecting bolt of seawater circulating pump gearbox in a nuclear power plant fractured during service. The fracture reasons of the bolt were analyzed by macroscopic observation, chemical composition analysis, hardness test, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the bolt had fatigue fracture, and the ferrite in the decarburized layer on the bolt surface reduced the surface hardness and wear resistance of the bolt. During the disassembly and assembly of the bolt, the folding or microcracks were generated by friction with the edge of the bolt hole, and these folding or micro cracks became the fatigue source of early crack formation and propagation of the bolt.