Abstract: AISI M3:2 high speed steels with or without niobium addition were prepared via spray forming, then subjected to forging processes. The effects of Nb on the microstructure and properties of the high speed steels were investigated by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. It is found that Nb addition can refine the microstructure of the as-deposited steels and decrease the size of M₂C eutectic carbides. The amount of MC carbides in the as-deposited steels increases with increasing Nb content. The tempering resistance and second hardening ability of the high speed steel are improved by Nb addition. With a 1% Nb addition, lots of bulk Nb-rich primary MC carbides can form, which becomes the main cause of the reduction in bending strength and impact toughness of the high speed steel. However, the best mechanical properties can be achieved in the presence of 0.5% Nb.