Abstract: The teeth of gear shaft in reduction box broke during operations. The breaking reason was analyzed by means of macroscopic examination, chemical composition analysis, mechanical property test, metallographic examination and so on. The results showed that the degree of stress concentration increased at the gear bottom and the fatigue strength of gear shaft reduced a lot when it formed steps between the tooth surface and tooth groove because of improper mechanical processing. The teeth was cracked under the affect of alternating stress, thus resulting in the early fatigue failure.