Abstract: After two long-time tests, the high pressure fuel pump was found to have cracks on the oil-out by fluorescence detection. The failure reasons of the fuel pump were studied by macroscopic observation, metallographic analysis, scanning electron microscopy, energy dispersive spectroscopy and micro-hardness analysis. The results show the crack initiated from the R corner of the oil-out and the crack is fatigue crack. The results of stress analysis show that there was stress concentration at the R corner. In addition, there was pitting corrosion at the R corner surface, increasing stress concentration and promoting the initiation of the fatigue crack.