The article examines the changes of the structural-phase states and the microhardness of the R6M5 steel surface layer after electrolytic-plasma nitriding. It is found that after electrolytic-plasma nitriding of the R6M5 steel surface, diffusion layer is formed, which is a nitrogen martensite. The phase composition of the diffusion layer varies depending on the nitriding temperature. An increase of R6M5 steel microhardness, depending on structural-phase state, is found out. The main factor, influencing the increase of microhardness of R6M5 high-speed steel with electrolytic-plasma nitriding, is the formation of nitrogen martensite with monophasic nitride Fe 4 N ( γ ′ -phase), as well as the formation of fine inclusions, hardening phases in the surface layers.