Abstract: The connecting bolts in a hydrogen reciprocating compressor were ruptured after woking normally for a period of time. The fracture cause of the bolts was investigated by fracture macro- and micro-morphology observation, chemical composition analysis, microstructure observation and mechanical property tests. The results show that the fracture mode of the bolts was low stress high cycle fatigue fracture. Under alternating stresses, some microcracks originated from the high stress concentration position at the root of the screw thread of the bolt. Improper heat treatment resulted in the mixed structure of tempered sorbite, upper bainite and ferrite, leading to the decrease of the strength and hardness of the bolts and the acceleration of fatigue crack propagation. Fianlly the fatigue fracture of connecting bolts occurred.