Abstract: 20MnCr5 steel gear was subjected to conventional vacuum carburizing heat treatment at 930℃. The microstructure, element distribution and microhardness of the carburized layer were studied by optical microscope, scanning electron microscope, transmission electron microscope, electron probe and microhardness tester. The results show that the microstructure of the carburized layer surface layer consisted of high-carbon acicular martensite and residual austenite. With increasing the distance from the surface, the acicular martensite changed to lath martensite. Strip-like chromium-rich carbides and spherical manganese-rich carbides were also precipitated in the carburized layer, and the carbides were dispersed. The surface hardness of the carburized layer was 860 HV, which was much higher than that of the substrate. With increasing the distance from the surface, the hardness of the carburized layer decreased.