Mo-Cr diffusion layer on carbon steels are prepared by double glow plasma surface metallurgy and then treated by ultra-saturated carbonization, quenching and tempering. The content of component is roughly that of high-speed steels (HSS). The results showed that the depth of coating is over 100μm and the content of Mo, Cr alloyed layer is about 20% and 10% respectively. Surface carbon content is over 2.0% after ultra-saturated carbonization. The carbides of the alloyed layer are fine and dispersed, without coarse eutectic ledeburite. X-ray diffraction showed that the carbides are M6C, M2C, M23C6 etc. SEM analyse indicated that the dimension of surface carbide is less 1μm. The abrasion experimental results showed that the relative wear resistance of the treated sample was 2.38 times as that of quenched GCr15.