Abstract: Several constant velocity universal joints produced by a company were fractured after quenching. The fracture causes of constant velocity universal joints were analyzed by means of macroscopic examination, chemical composition analysis, macrostructure examination, metallographic examination and so on. The results show that because of the band segregation in core area of the raw material and improperly heat-treatment, the source of the cracks formed in the core area during the quenching process, and finally caused the fracture of the constant velocity universal joints under the action of stress.