A hardfacing composition for downhole well tools, such as earth-boring bits, contains sintered ultrahard particles. The ultrahard particles consist of tungsten carbide grains, cobalt and vanadium. The ultrahard particles are dispersed within a matrix metal of iron, nickel or alloys thereof. The composition may also have sintered tungsten carbide particles of a larger size than the ultrahard particles. The ultrahard particles have a greater hardness than the sintered tungsten carbide particles. The ultrahard particles and the sintered tungsten carbide particles may be in a spherical pellet form. Other hard metal particles may be in the composition.