Abstract: The fastening bolt made of 40CrNiMoA steel for a marine engine piston fractured after approximately 5 000 hours of service. Methods such as macroscopic observation, chemical composition analysis, mechanical property testing, scanning electron microscopy and energy dispersive spectroscopy, and metallographic examination were employed to analyze the cause of the bolt fracture. The results show that the fracture of the bolt was fatigue fracture. The corrosion medium contained sulfur, which caused corrosion pits at the junction between the transition R of the bolt head and the shank. There was stress concentration at this location, and under the alternating stress generated by the reciprocating motion of the piston, microcracks initiated at this point. The cracks propagated via fatigue, ultimately leading to the fracture of the bolt.