Abstract: During the assembly of the drive gear shaft for the rear axle spiral bevel gear, thread fracture occurred at the surface near the R area. The fracture surface is flat with metallic luster. The crack initiates from one side of the surface and expands radially. A black area exists on the surface layer at the crack source. The fracture surface clearly shows two stages of expansion. Comprehensive analysis based on fracture morphology, metallography, hardness,and energy spectrum indicates that the thread fracture is mainly caused by unqualified metallographic structure on the thread surface during heat treatment, which is acicular martensite. The surface hardness of the thread is uneven and fails to meet the technical requirements. There are defects of poor surface roughness in the machining process,which easily leads to crack initiation and fracture at the R corner due to excessive and uneven stress. By changing the thread heat treatment process from the original overall carburizing quenching + high-frequency induction annealing to applying anti-carburizing agent first and then performing high-frequency induction annealing on the thread during heat treatment, it ensures that no stress concentration occurs at the thread after heat treatment, avoiding thread fracture failures.