Abstract: The valve cover bolts(20CrMo1VTiB) of a 600 MW subcritical unit fractured. The reasons for bolt fracture were studied by macroscopic morphology, chemical composition, metallographic structure, SEM and mechanical properties test. The results show that the carbides precipitated in the grain and grain boundaries of the bolts gradually accumulate and grow up during long-term high temperature service, and the carbides distributed along the grain boundaries initially form a network, which significantly reduce the impact toughness of the bolts. At the same time, due to improper pre-tightening control and stress relaxation of bolts, the bending stress of the bolts increase, finally, the fatigue fracture occurs at the root of the first thread where the stress concentration is maximum. Based on the analysis of the bolt fracture, the suggestions are put forward.