Spray forming is a casting process, by which the molten metal is directly converted to a solid bulk with unique characteristics. When used in the production of high speed steel, spray forming materials typically present microstructures composed of refined polygonal grains, uniformly distributed carbides and low levels of micro and macro-segregation. The mechanical properties of the spray forming high speed steel are usually between ones made by powder metallurgy, casting and wrought. It can be considered as a cost saving alternative for large-scale industrial production of high-speed steels. But for high-speed steel produced by spray forming, its shortcomings can't be ignored: i.e. once being subjected to the same heat treatment, compared with the steels made by PM and CW process, the spray forming one often shows lower hardness. Focusing on the solution of aforementioned disadvantages, the effect of different heat treatments on the microstructural evolution and hardness variation was assessed for the spray forming M3 high-speed steel, and the adopted heat treatment involved quenching and tempering at different temperatures separately. Meanwhile the reasons for the low hardness of the spray-formed M3 high-speed steel were also discussed. The results show that SF M3 high speed steel quenched below 1230℃ can still ensure relatively fine grain size and uniform size distribution of carbide particles; Setting the tempering at 560℃, in case the quenching temperature raises from 1200℃ to 1230℃, after being quenched + tempered, the hardness of spray-formed M3 high-speed steel can be greatly improved. It is believed that the low hardness issue may be ascribed to the fact: the formation of a large number of MC type carbides with inhomogeneous composition in the spray forming atomization stage, the MC type carbides can't be fully dissolved in the steel matrix when quenching at lower temperature during the heat treatment process, resulting in insufficient amount of carbon and alloying elements in the matrix. Therefore, the low hardness issue is caused by the inability to fully exert the secondary hardening effect.