Abstract: As-cast and as-spray formed billets of M3 high speed steel were produced through traditional casting and spray forming, respectively. The effect of cooling rate on the microstructures of the cast and spray-formed steels, the relation of heating temperature to the decomposition of M₂C eutectic carbide, and the microstructural evolution of the cast and spray-formed steels after hot working were studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXA), and X-ray diffraction analysis (XRD). The as-cast structure contains coarse primary dendrites and M₂C eutectic carbide, but the spray-formed one consists of equiaxed grains and fine carbides. High cooling rate can suppress the carbide precipitation and grain growth. Elevated temperatures are favorable to the decomposition of M₂C eutectic carbide, but an exorbitant temperature results in the growth of M₆C carbide, which will cause the properties of the steel to deteriorate. A desired deformation microstructure of the spray-formed steel can be obtained by appropriate preheating and hot deformation.