Abstract: Several cracks were found among the teeth of a manual thin walled drive spur gear before service, the cracks morphology and fracture characteristics were analyzed, and the failure causes were identified during the design, processing, assembly and other processes. The results show that the feature of the cracks in the spur gear are vibration fatigue. Because of improper clamping fixture, an extra axial component force is imposed on the spur gear during abrasive machining, which resulted in resonance vibration of the gear. By changing the fixture structure and adopting the clamp wheel disk clamping way, this type of defect can be avoided effectively.