Abstract: The depletion of alloying elements and composition demixing in surface alloyed layer on the 20^# steel in process of double plasmas suface alloying hastelloy C-2000 had been studied. The results showed that the depletion of alloying elements were related to distortion gradient in surface alloyed layer and phenornena of antisputter, and the compostion demixing emerged as a result of different sputter character of element Ni,Cr,Mo.