Abstract: In order to analyze the cause of bolts fracture for main bearing of a diesel engine generator, the chemical composition, mechanical properties, macro and micro morphology of the fractured bolts were observed and detected. The results show that the chemical composition of the bolts, except the copper element, meets the design standard, the tensile strength is about 18% higher than the design standard, and the hardness is about 5% higher than the upper limit of the design requirements. The fracture of bolts is independent of their material, but their high strength and hardness make fretting crack propagation easily, which leads to fatigue fracture. There are several fatigue crack sources in the fracture of the bolts, and the area of crack propagation zone is larger, which show the fracture characteristics of fretting fatigue. Fretting fatigue can be identified as the main cause of bolt fracture.