Plasma radical nitriding has been performed to harden the surface of SKD 61 steel for 1- 10 hours at temperature range of 450-550°C. A NH radical, which has played a key role to produce a nitrogen diffusion layer without the formation of the brittle compound layer, has been generated in a gas mixture of NH3 and H2 . One of the main advantages of the plasma radical nitriding is to improve the surface hardness by maintaining the roughness of the initial polished surface. The microstructures and material properties of the radical nitrided layer have been characterized in order to investigate the effects of various radical nitriding processing parameters. The hardness and surface roughness of the hardened layer were compared between two processes. In addition, PVD CrN coating has been deposited on both the radical nitrided substrates and conventional nitrided substrates by an arc ion plating (AIP) technique. The effect of two different of plasma nitriding treatments on the adhesive strength of the coating layer on the substrates was also investigated.