This paper deals with fatigue behavior of ion-nitrided coil spring. It is found that fatigue limit can be significantly increased by ion-nitriding. Ion-nitribed specimen which is treated at 550.deg. C for 5 hours improves in the fatigue limit by 30 percent in comparison with that of non-nitrided specimen. On the other hand, the value of spring constant K has nothing to do with nitriding time and temperature in this experimental range. Besides fatigue behavior, the following effects are discussed such as compound layer, diffusion layer, hardness distribution and their relations.