In this study the improvement of mechanical properties of ion-nitrided SACM-1 steel was investigated by analysing microstructural developments and hardness increase in the nitrided surface layer. Specimens were quenched at 570℃, which is lower than the eutectoid temperature(590℃) of Fe-N binary system after nitrided at temperature of 460-570℃ for 2-8 hours under constant pressure of 8 torr. The depths of diffusion and compound layers were appeared to proportional to the root mean square time of nitriding. And the hardness showed the maximun value Hv=1200 for the specimen nitrided at 530℃. Hardness distribution of the ion-nitrided steels were increased by diffusion treatment below the eutectoid temperature of the Fe-N binary system. A prolonged heat treatment below the eutectoid temperature was attributed to the increase in the depth of diffusion layer at the expense of the decrease in surface hardness of the ion nitreded steel.