Abstract: During the maintenance of a power plant, it was found that the connecting bolts of the oil motor base fractured. The fracture causes of the connecting bolts were analyzed by means of macro analysis, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope analysis and thread root inspection. The results show that the fracture type of the connecting bolts of the oil motor base was high cycle fatigue fracture, which belongs to brittle fracture caused by fatigue. The small crack defects in the chamfer of the connecting bolts made the stress concentrated and produced many crack sources. The existence of granular ferrite in the connecting bolts microstructure reduced the fatigue strength of the connecting bolts and finally led to the connecting bolts fracture.