Abstract: Joint bolts which connected the exhaust pipe and the cylinder on the LLS diesel engine generator set fractured in a nuclear power plant unit. By means of macroscopic and microscopic analysis, chemical composition analysis, microstructure analysis, mechanical property test and so on, the fracture reasons of bolts were analyzed. The results show that:the failure mode of the GH2036 alloy bolts was intergranular brittle fracture; the failure reasons were that the heat treatment process of the bolts was improper, resulting in the precipitation of lamellar brittle phase along the grain boundary; the phase weakened the grain boundary, which induced that the intergranular brittle fracture occurred to the bolts under the action of high temperature and stress.