Abstract: The worm-gear connecting bolts of refueling machines of a nuclear power plant, with material of 35 steel and strength grade of 10.9, fractured after the device running for about a decade, and means such as macro examination, chemical compositions analysis, hardness test, metallographic examination, fracture SEM analysis and so on, were used to analyze the fractured bolts. The results show that the fracture property of the bolts is two-way bending fatigue fracture. Surface decarburization of the bolts and stress concentration at the bolt thread neck decreased the fatigue strength, which resulted fatigue cracks formed here. By comprehensive analysis and force estimating, it was concluded that the main reason for the fracture is that there is a big gap between the bolt shank parts and their matching internal gear bolt holes, which made the force state and force size of the bolts changed early and finally resulted in fatigue fracture of the worm-gear connecting bolts.