Abstract Ion-nitriding for S45C, SCM435 and pure iron was carried out using an Al subsidiary cathode at 823 K for 10.8 ks in 665 Pa mixed gas of 30% N 2 – X % H 2 – Y % Ar. The nitriding layers formed on each steel were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), microscopic observation and microhardness measurement. The surface hardness of S45C was increased with Ar contents up to 20% at 30% N 2 . After the ion-nitriding, iron nitride phases of e-Fe 2–3 N and γ′-Fe 4 N were found on the surface of S45C and SCM435, and γ′-Fe 4 N phase was found on the pure iron. From the results of XPS analysis, Al was found as a combined form with nitrogen in the nitrided layer. Using an Al subsidiary cathode, the surface hardness and thickness of the compound layer were greater than those without. The surface hardness of S45C, SCM435 and pure iron was Hv 820–890, >Hv 1000 and Hv 240, respectively. These may be due to the presence of the sputtered Al atoms, which were deposited and/or diffused into the iron nitrides and then increased the lattice strain.