Abstract: The fracture failure causes of the piston rod used in a hydrogen compressor were analyzed based on macrography, micrography and chemical composition analysis, microstructure observation, tests of tensile and impact properties and hardness. Results show that failure of the the piston rod of hydrogen compressor was due to fatigue fracture. When the piston rod proceeded thermal refining, the cooling rate in quenching was too slow and the tempering temperature wasn′t high enough, so the obtained microstructure was the mixture of heterogeneous martensite, pearlite and sorbite in stead of homogeneous sorbite. Therefore the comprehensive mechanical properties of piston rod declined and led to an early occurrence of fatigue fracture.