Abstract: The effects of quenching and tempering processes on microstructure and hardness of X20CoCrWMoV10-9 hot-work die steel used in new magnesium alloy pressure die casting machine were studied by using the OM,SEM,EDS and XRD.Then the heat treatment process was determined.The results show that the optimized heat treatment of the steel was suggested that the steel was annealed at 840 ℃ for 8 h,then solution-treated at 1 100 ℃ for 1 h and oil quenched,followed by aged at 700 ℃ for 2 h.The microstructure of the steel treated with the process was consisted of tempered martensite,intermetallics μ phase,carbide M23C6 and M6C.The hardness of the steel reached to 51.5 HRC after quenched at 1 100 ℃ for 1 h.Due to the precipitation of intermetallics and alloy carbides in the steel during the tempering treatment at 550-650 ℃,the steel presented high tempering resistance.