Abstract Powder-metallurgy processed high-speed steels containing up to 5 wt.% of Nb were fabricated by nitrogen gas atomization of elemental powders followed by hot isostatic pressing and hot working. Microstructural changes, particularly the carbide formation during the heat treatment and the resulting mechanical properties were investigated. After the heat treatment, the primary carbides were identified as MC and M6C-types. With Nb addition, the total volume of primary carbides increased from 16 to 23 vol.% and the volume fraction of MC-type carbide in the primary carbides also increased. The chemistry of the MC-type carbide changed from a V-rich to a V–Nb-rich carbide with increasing Nb content. Hardness significantly increased to a maximum of Rockwell C66 with 1 wt.% Nb and then slightly decreased with further Nb additions. Bend strength, on the other hand, gradually decreased with increasing Nb content. Fracture toughness is almost inversely proportional to the hardness and wear and softening resist...