Abstract: The nitriding heat treatment of carburized G13Cr4Mo4Ni4V steel was carried, the effect of nitriding time on the microstructure of the composite layer, carburized layer and core of the steel, as well as the hardness, was studied.Results show that the composite layer and carburized layer of G13Cr4Mo4Ni4V steel after nitriding are composed of carbides, tempered martensite and a small amount of retained austenite, and the core region is composed of lath martensite and a small amount of carbides.After nitriding for 90 h, the Rockwell hardness of the steel surface increases, but the fluctuation also increases.The microhardness of the steel after nitriding for 50 h can reach 960 HV, the thickness of the nitriding layer is 0.17 mm, and the thickness of the effective carburized layer is 1.67 mm.The microhardness of the steel after nitriding for 90 h can reach 906 HV.The thickness of the nitriding layer is 0.19 mm, and the thickness of the effective carburized layer is 1.26 mm.The decrease in the thickness of the effective carburized layer is related to the inward diffusion of the carbon element and the resultant low carbon content in the matrix during the nitriding process, and the decrease in microhardness is related to the precipitation of network carbides which reduces the carbon content of the matrix.