Abstract Plasma nitriding was performed on AISI 630 martensitic precipitation hardening stainless steel samples at 400 V with a gas mixture of H 2 and N 2 for 19 h with changing N 2 percentage, temperature and adding various percentages of CH 4 . After treatment the behavior of the surface layer was investigated by optical microscopy, X-ray diffraction, GDOES analysis and micro-hardness testing. A potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. With increasing N 2 percentage from 15% to 35% at 380 °C, the thickness of the expanded martensite (α′ N ) layer increases to about 20 μm and surface hardness also increases to about 1280 HV 0.05 but the corrosion behavior is worse than the untreated sample. When nitrided with 25% N 2 , increasing nitriding temperature from 380 °C to 460 °C also increases the α′ N layer thickness to around 38 μm and surface hardness to around 1270 HV 0.05 (at 460 °C), but decreases the corrosion resistance. Thus in order to improve the corrosion resistance, various amounts of CH 4 (0% to 6%) were introduced in the nitriding atmosphere at 400 °C with 25% N 2 percentage. Increasing CH 4 percentage slightly decreases the α′ N layer thickness but greatly improves corrosion behavior. Moreover, with 4% CH 4 percentage, the best corrosion behavior is obtained which also has around 11 μm α′ N layer thickness and about 1290 HV 0.05 surface hardness. Therefore, the conclusion can be drawn that, the highly suitable condition for plasma nitriding for a thicker expanded martensite layer, higher surface hardness and good corrosion resistance is plasma nitriding at 400 °C with 25% N 2 and 4% CH 4 for 19 h.