In this paper, cobalt, tungsten and molybdenum were co-diffused into the surface of T8 steel by double glow discharge technique. A carbide layer with M6C＋MC was formed on T8 steel. Under the carbide layer was a diffusion layer of W,Mo,Co. Hardness of carbide layer is high over 1200~1400 HV0.025 and can improve to 1400~1600 HV0.025 after solution and ageing treatment. Hardness of the diffusion layer is only 400~550 HV0.025, but it can improve to 800~1000 HV0.025 after solution and ageing treatment. The carbide layer combides with the substrate in metallurgy and can not peel off from the substrate. The carbide layer on T8 steel possess same good tribological properties of M2 steel.