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X-ray diffraction (XRD) and X-ray photoelectron spectroscopy were used to study the structure and the element chemical states of plasma-based ion implantation (PBII) nitriding M50 steel. An ~70-nm-thick oxygen-rich layer was formed in the surface after PBII treatment, where C and V elements were absent, Cr and Mo elements were in the manner of oxide, and Fe element was in the manner of Fe3O4 and iron nitride. XRD patterns revealed that the Fe16N2 phase was formed in the subsurface of the nitrided layer. Four different chemical states of nitrogen with binding energies of 404.1, 399.2, 396.7, and 394.3 eV were found in sequence with sputtering time increasing, which were confirmed to belong to NO bonding, Fe3(N, O) [or Fe4(NO)], Fe3N (or Fe4N), and Fe16N2.