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40CrNiMoA steel was nitrided by arc plasma-assisted nitriding (PAN) process at 430 °C, 1 Pa for 3 h with different N2/Ar ratios. Their structures, micrographs, microhardness profiles and tribological properties were investigated by X-ray diffraction (XRD), optical microscope, SEM, Vickers microhardness tester and pin-on-disk tribometer, respectively. All of the nitrided layers were primarily consisted of the γ′-Fe4N and ε-Fe3N. The thickness of compound layers increased with the N2/Ar ratio increasing. The maximum thickness of the compound layer was about 6 μm after nitriding, while the thickness of nitrided layer reached about 230 μm. The surface microhardness and wear resistance of the steel were significantly improved. Its maximum surface microhardness enhanced as 3 times (approx.) as that of the substrate. The wear mechanism changed from adhesive wear to abrasive wear. In addition, the nitrided sample treated at N2/Ar ratio of 75%:25% had the optimum abrasion resistance and lowest wear rate. [ABSTRACT FROM AUTHOR]