Abstract: Three kinds of common and typical gear steels were carburized and heat treated, and the surface carbon contents of three kinds of steels after different carburizing times, and the hardness, microstructure of carbonization layers before and after heat treatment were studied. The results show that the alloy coefficients of three kinds steels were, from high to low, 20CrMnMo steel, 17CrNiMo6 steel and 20CrNiMo steel. The surface carbon contents increased with the increase of alloy coefficients under the same carbonization conditions. The surface hardness of three kinds of steels were all over 45 HRC before high-temperature tempering, and decreased after tempering. The microstructure before high-temperature were all nonequilibrium mixed microstructure of martensite and retained austenite.