The structure and hardness of SCrWMoV steel with multiple types of ultra fine carbides are studied after annealing, quenching and tempering in this paper. The results show that multiple types of carbides M3C, M7C3, M23C6, M6C and MC were observed in the annealed steel. Nucleation and coalescence of new carbides, partial dissolution of original carbides in phase region during annealing at 800-840 °C, result in ultra-fine carbides. Average size of the carbides is 0.33~0.34(im in the steel annealed at 800-840°C. Because M3C and M23C6 dissolve easily in austenite, the high hardness HRC63-65 can be obtained by quenching at 840-860 °C. Un-dissolved carbides M6C and MC (VC) can effectively prevent the coarsening of austenitic grain, and conduce to obtain very fine martensite. The retained austenite can be easy to decompose during tempering at low and middle temperature due to the precipitation of multiple types of carbides and the good tempering-resistance of the steel is obtained. The microstructure and property of the steel after heat treatment can be accurately explained by calculating based on phase equilibrium thermodynamic. The structure and hardness of SCrWMoV steel with multiple types of ultra fine carbides are studied after annealing, quenching and tempering in this paper. The results show that multiple types of carbides M3C, M7C3, M23C6, M6C and MC were observed in the annealed steel. Nucleation and coalescence of new carbides, partial dissolution of original carbides in phase region during annealing at 800-840 ° C, result in ultra-fine carbides. Average size of the carbides is 0.33-0.34 (im in the steel annealed at 800-840 ° C. Because M3C and M23C6 dissolve easily in austenite, the high hardness HRC 63-65 can be obtained by quenching at 840-860 ° C. Un-dissolved carbides M6C and MC (VC) can effectively prevent the coarsening of austenitic grain, and conduce to obtain very fine martensite. The retained austenite can be easy to decompose during tempering at low and middle temperature due to the precipitation of multiple types of carbides and the good tempering-resistance of the steel is obtained. The mic rostructure and property of the steel after heat treatment can be quite explained by calculating based on phase equilibrium thermodynamic.