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The polished 9Cr18Mo martensitic stainless steel was implanted with nitrogen ions at a fluence of 2×1017 ion/cm2. To investigate the surface mechanical properties of the 9Cr18Mo steel samples after nitrogen ion implantation, the surface nanohardness and tribology properties of the original and nitrogen ion implanted steel samples were investigated. At the same time, their surface morphologies were also examined. The results show that nitrogen ion implantation would improve the surface hardness and lower the friction coefficient and wear rates of 9Cr18Mo steel samples significantly, while it does not change their surface morphology and surface roughness. To elucidate the strengthening mechanism of nitrogen ion implantation, the surface structures and components were studied by X-ray diffractometer and X-ray photoelectron spectroscopy. The results indicate that nitrogen ion implantation region can be divided into two layers. One is a surface layer of around 20 nm thickness, which is composed of metal nitrides and solid solution phases besides plenty of carbon atoms and the carbon atoms accumulate at the surface in a form of CrxCy phases. The other is the subsurface layer below 20 nm depth, which is mainly composed of metal nitrides. The surface components and structures have significant contributions to the improved mechanical properties.