Abstract: The fracture reason for the principal axis gear of car was caused by bend stress according to the failure analysis results. Cracking initiated on spline due to local over plastic deformation in the process of cold leveling,because spline has the maximum bend stress for minimum section size. Impact load on the principal axis led to final fracture during nut assemblying and car running. The heat treatment deformation rule for the part was analyzed and discussed,the suggestion of improved manufacturing process was proposed.