Abstract: Optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and spark discharge atomic emission spectrometer were used to analyze the fracture reason of the piston rod for a new hydrogen compressor. The results manifested that the chemical composition and metallographic structure of 42CrMo steel accorded with the standards. The failure of piston rod was caused by the fracture fatigue. A large number of alumina inclusions, sharp and aggregated, with the size largerer than 10 μm, were present within the piston rod. Under the action of alternating loads, stress concentration was formed where these inclusions were gathered, and fatigue cracks were formed. This is the main reason for the fracture of the piston rod.