Abstract: Some fundamental aspects of the double glow discharge plasma surface alloying process(Xu-Tec. process) were discussed and modeled. The sputter of cathode material surface, the emission of sputtered atom flux from the source cathode, the creation of freely migrating point defects in the workpiece surface and their diffusion into matrix was presented and calculated. On the basis of a series of rational and acceptable assumptions and simplifications, a set of pantal differential equations for atomic fluxes were contracted and solved to describe the surface alloying process, and to predict the evolution of depth-concentration profile of the alloying element. The theoretical prediction is in accordance with the experimental results of samples.