Abstract: The fracture reasons of connecting bolt of piston head and piston skirt in a diesel engine for ship were analyzed by OM,SEM and EDS etc.The connecting bolt was fractured by fatigue with many fatigue sources,which entirely located at the fang surface of screw threads and resulted from the low surface machining precision of screw threads.The existence of reticulation and mass shape ferrite,decarbonization at the screw thread surface layer which induced decline of fatigue strength of connecting bolt were also the important reasons of fracture.