This research tries to improve the plasma surface W, Mo alloying process by adding inlet methane aside from the original inert gas argon. The carbon and hydrogen particles are incorporated into the ion sputtering, ionization, surface activation and diffusion processes. The W, Mo atoms sputtered from the target diffuse into the surface of the substrate at the same time with the diffusion of carbon atoms. So the synergism of the alloying process and the carburizing process is established in this way. The hydrogen atoms participate the reduction and activation process on the surface of the target and the substrate. The surface HSS combines with substrate via metallurgical bonding and the carbides are all secondary carbides formed at lower temperature during solid state diffusion. These carbides are very soluble to the austenite. This makes the alloy elements fully functional. The carbides with granular shape and distributes homogeneously on the matrix are very fine. No coarse ledeburite eutectic carbide exists. After the co-alloying process of W, Mo, C, direct quench or quench at lower temperature can be applied.