The purpose of this study is to investigate the effect of cavitation peening on the rolling contact fatigue life of steel gears. The steel rollers and gears cavitation-peened under different conditions were fatigue-tested using a roller testing machine and a gear testing machine. After the cavitation peening, the surface hardness and the surface compressive residual stress of the steel rollers and gears were increased. On the other hand, the surface roughness was not increased so much. As a result, the fatigue life of most cavitation-peened rollers and gears was longer than that of the non-peened ones. In particular, the fatigue life was significantly improved by using a modified nozzle for a high-speed water jet to obtain a greater peening effect. It is suggested from these that the cavitation peening is good for the increase in fatigue life of steel gears, because the peening effect is obtained with less increase of the surface roughness.