The paper shows the possibility of plasma electrolytic polishing of the steel surface after its chemical-thermal treatment in a single technological process. Positive results of the plasma electrolytic polishing are obtained for carbon steel after its treatment in borid acid, nitrate and ammonium chloride solution. In the process of PET, three processes simultaneously occur on the steel surface: oxidation, diffusion saturation with nitrogen and boron, and anodic dissolution of steel. An oxide layer is formed on the surface of the steel as a result of oxidation. Under the oxide layer, the diffusion layer is detected, which is the result of nitrogen and boron diffusion with the formation of iron nitrides and a solid solution of nitrogen and boron. The formation of surface topology is shown to determine by the competition between two processes: oxidation and anodic dissolution. The uneven formation of the oxide layer occurs during oxidation. On the contrary, anodic dissolution promotes smoothing of the surface. The reduction of the surface roughness and corrosion current density of plasma electrolytic treatmented steel by plasma polishing using mode of current interruption for 2 min without changing the structure of the diffusion layers is shows.