Abstract: Crack defect appeared on a tooth surface of a gear made of S40C steel after working for a certain time. The cause of the defect was analyzed by means of macro & micro analysis, metallographic examination and microhardness test. The results show that the crack defect of the tooth surface was caused by contact fatigue, and the root cause of the local contact fatigue crack is grinding burn on tooth surface, which significantly reduced the hardness of tooth surface, and changed the gradient distribution of tooth surface strength. As a result, contact fatigue crack occurred under the action of cyclic engagement. Furthermore, the influencing factors of the hardness reduction of induction hardened gear tooth surface are described in detail. Combined with the relationship between the gear strength curve and the shear stress curve under different meshing conditions, several common phenomena of gear peeling and failure mechanism are discussed.