Abstract: In this study, the gas nitriding behavior of 31CrMoV9 steel was investigated. The nitriding process was performed in ammonia gas atmosphere at 510 oC, for 16 and 100 h, and 590 oC, for 4 and 36 h. Optical microscopy was used for examining the morphology of the nitrided layer. Microhardness and compositional depth profiles were obtained using cross-sectional samples. X-ray diffraction method combined with repeated chemical-etching surface layer removal for depth profiling was used to determine the variation of phase composition and residual stress as a function of depth. Optical microscopy examinations of the cross-section of nitrided specimens revealed a two-layer structure of the nitrided zone: compound (or white layer)/diffusion zone. It was found that nitriding for 36 h at 590 oC brings about a nitriding zone reaching to a depth of about 700 μm. The highest surface hardness, about 800 HV0.1, was obtained for specimens nitrided at 510 oC. X-ray diffraction depth profiling confirmed the presence of Fe3C, CrN, α-Fe, and residues of γ'-Fe4N (up to a certain depth) in the diffusion zone. A pronounced dissolution of Fe3C in the near-surface region and build up towards the nitriding front was observed, while CrN nitrides exhibit a more or less constant value across the diffusion zone. All residual stress depth profiles are characterized by a maximum compressive stress occurring at some depth below the specimen surface. Results indicate that the maximum compressive stress decreases with nitriding time and its depth increases with nitriding time due to proceeding homogenization of the specimen, the dissolution of Fe3C in the near-surface region, which is also supported by overaging of the formed CrN precipitates. Increasing the temperature to 590 oC contributed to a diffusion zone with more than 50% lower maximum compressive stress, resulting from the redistribution of carbon in the diffusion layer and faster overaging.