Plasma radical nitriding was performed to harden the surface of SCM 440 steel for 1-10 hours at temperature range of 450-550°C. No compound layer was formed during this process except the experiment carried out at 500 for 10 hours. A diffusion depth increased with increasing treatment temperature and time ( up to about 250). The surface hardness of radical nitrided layer was two times higher than that of the untreated surface. The main phase produced in the diffusion zone was identified to be γ΄-Fe4(N,C). The residual stress of the diffusion layer also increased with increasing treatment temperature and time due to the increase of precipitates.