Abstract: The effects of different temperatures at 550-650 ℃ on the microstructure and mechanical properties of 42CrMo gear steel were studied by SEM and tensile testing machine. The results show that the microstructure of tempered 42CrMo steel is mainly ferrite, cementite and a small amount of retained austenite. With the increase of tempering temperature, the amount of carbide precipitated in the test steel first increases and then decreases gradually, the size of the carbide becomes smaller and smaller, and the ferrite gradually changes from strip shape to block shape. With the increase of tempering temperature, the tensile strength of the test steel decreases gradually and the elongation increases gradually. The tensile strength of tempered steel at 550 ℃ is highest, reaching 1152 MPa, and the elongation of the tempered steel at 650 ℃is highest, reaching 17.2%.