From the point of view of nitriding treatment(570°C), Cu precipitation is also occurred in the steel containing 1.3mass%Cu at this temperature. In this study, the behavior of nitriding in ultra low carbon steels containing Cr and/or Al and/or V are investigated. The following results are obtained : (1)After nitriding treatment the distribution of vickers hardness(Hv) differs in added nitriding element. (2)Profile of hardness in thickness is resulted from mainly precipitation hardening of nitride. (3)The high fatigue limit of nitrided steel is occurred by residual stress in vicinity of the surface. (4) Nitride precipitation is promoted by Cu precipitation that occurs in early stage of heat treatment.