Abstract: The effects of quenching and tempering on the microstructure and mechanical properties of high carbon Cr12MoNiV4 die steel were studied by OM, impact test and hardness test. The results show that the microstructure of quenched high carbon Cr12MoNiV4 die steel are mainly martensite, retained austenite and residual carbide. With the increase of quenching temperature, carbides gradually dissolve in the microstructure of the test steel, and the hardness of the test steel first increases and then decreases. The hardness of the test steel quenched at 1130 ℃ reaches the maximum of 64.1 HRC. In the tempering temperature range of 400-600 ℃, the test steel generates secondary hardening after tempering. The hardness of the quenched test steel at 1130 ℃ reaches its maximum after tempering at 500 ℃, and the impact work reaches its maximum after tempering at 600 ℃.