Abstract: Combined teeth of synchronizer gear sleeve of an automobile fractured during test process. The causes of fracture on combined teeth were analyzed by means of chemical composition analysis, scanning electron microscopy analysis, energy spectrum analysis, metallographic examination, hardness and effective depth of carburized and hardened layer test. The results show that the fracture of combined teeth was related to the intergranular oxidation at the root of teeth. The intergranular oxidation reduced the binding force of grain boundary, which made the generation and propagation of quenching microcracks along the R-angle of teeth bottom with the stress concentration under the joint action of internal stress and external load, and finally led to the fracture of combined teeth.