Warm Shot Peening is a shot peening process within warm temperature range. However, the effect of warm shot peening to fatigue strength was not certain and no substantial study has been made.The requirements of coil spring with higher fatigue strength and sag resistance have been increasing to obtain the mass saving of vehicle. While several new spring materials have been developed in recent years, the essence of those developments were to make the strength(hardness) of spring higher, so-called high strengthened spring. As for high strengthened spring, the shot peening process becomes further essential to relieve the increased notch sensitivity due to higher strength. However, for the shot peening process to high strengthened spring, the shot with higher hardness is also required. This can decrease the life of both shot and shot peening equipments.Warm shot peening under the tempering temperature may become one solution to those problems, because the hardness of spring can be temporarily reduced only during the shot peening process. In this paper, the following experimental results concerned with the effect of warm shot peening, are summarized. 1The relation between spring fatigue life and hardness under one warm shot peening temperature. 2The relation between spring fatigue life and shot peening temperature. 3Effects of residual stress, coverage, and surface hardness to fatigue life.