Abstract: The fracture reasons of the bolts connecting the wheel hub and spokes of a reduction gear in a nuclear power plant were analyzed by macroscopic examination, chemical composition analysis, mechanical property testing, metallographic examination and scanning electron microscopy (SEM) analysis. The results show that the failure form of the bolts was multi-source bidirectional fatigue fracture which was mainly related to the high shear stress caused by the spoke slide after the bolt looseness. A lot of super-sized brittle silicate inclusions existing in the raw material might accelerate the formation and propagation of the fatigue cracks.