Abstract: M3 high speed steel (HSS) with or without niobium addition was prepared by spray forming, and the effect of niobium on the microstructure of M3 HSS was investigated by scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD) analysis, differential scanning calorimetry (DSC) and optical microscopy (OM). Finer and uniformly-distributed grains without maerosegregation appear in the as-deposited HSS, and the addition of niobium tends to shift the precipitation of primary MC-type carbides to a higher temperature. Plenty of refined niobium-containing MC carbides precipitate as isolated sphere particles in the grain boundaries before onset of eutectic reaction. Niobium mainly appears in primary MC-type carbides by consuming carbon, which suppresses the degree of eutectic reaction to a large extent, leading to the decrease in both amount and size of M₂C lamella carbides. The refined metastable M₂C carbides in Nb-containing HSS can be completely decomposed easily during hot deformation, and the homogeneity of carbides in Nb-containing HSS is improved. The wear-resistant property of Nbcontaining HSS is superior to that of M3 HSS at a lower load or temperature. But this gap is reduced with temperature rise. The main reason is that the large number of refined primary MC carbides can improve the abrasive wear resistance of HSS, but have no effect on its oxidation resistance.