Abstract: The fracture occurred in the working process of 20CrMnTi steel gear in a transfer case. The fracture causes of the gear were analyzed by means of macro analysis, micro analysis, chemical composition analysis, hardness test, depth measurement of hardened layer, analysis of non-metallic inclusions and metallographic examination. The results show that the residual tool marks on the tooth surface led to stress concentration. Under the action of periodic load, the fatigue crack source firstly formed in the deep part of the residual tool mark, then the crack gradually expanded, and finally the gear fatigue fracture occurred.