Abstract: The investigation about influence of cobalt on properties of superhard W-Mo and Mo system high speed steel (HSS) containing 1% V had observed that the secondary hardness rised with the increasing of cobalt, and the strength and toughness could be increased in the range of 3%~5% Co. When cobalt content arrived to 8%, the brittleness would be appeared. A new type of superhard high speed steel, Co3N, with low cobalt content and the same hardness,red-hardness, strength, toughness and erindability of M42, was developed.