Abstract: The home bolt for changeover portion of combustion engine 9FA was fractured in a short timerunning.A comprehensive analysis to find the fracture reason which was based on chemical composition analysis, mechanical property testing and metallographic examination was made on the bolts.The results showed that theroughness edge of bolt screw thread made stress concentration and caused to micro-crack.The micro-crack extendeduntil the residual cross-section could not subject to the load and then made to fracture that caused by hot stress in thecontinuously hot recycle process.