Abstract: An output gear shaft fractured during test and the fracture surface had heavily worn. The failure mode of the shaft was analyzed and it is found that the failure mode is rotary bending high-cycle fatigue fracture. The chemical composition, mechanical properties and microstructure of the shaft are all in accordance with the requirement, but the fillet radius is smaller than the requirement. In order to find out the cause for the fatigue failure, the stress under the measured fillet radius and the required fillet radius was calculated, and the rotary bending fatigue limitation strength under different fillet radius was obtained. It is found that the actual fatigue limitation strength of the fractured shaft is close to half of the required fatigue limitation strength. Under the effect of working stress, the actual fatigue life of the fractured shaft decreased significantly, and early failure occurred to it during service.