Abstract: In the finish turning process of a 45 steel gear shaft after heat treatment, there were longitudinal hair cracks on the surface of the gear shaft. The causes of the hair cracks were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the hair cracks on the surface of the gear shaft was caused by the accumulation of sulfide inclusions. The sulfide inclusions accumulated in the heat treatment process caused stress concentration of the gear shaft, resulted in quenching cracks extending along the inclusions and finally formed the hair cracks.