In order to investigate the effect of cavitation peening on surface durability of steel gears, the case-hardened steel rollers and gears cavitation-peened under different conditions were fatigue-tested using a roller testing machine and a gear testing machine. The hardness and the compressive residual stress of the test rollers and gears were increased by the cavitation peening. On the other hand, their surface roughness was not increased so much. In the roller tests, the fatigue life of all the cavitation-peened rollers was longer than that of the non-peened roller. In the gear tests, the surface durability of the test pinions was improved by the cavitation peening. Judging from the relationships between the maximum stress amplitude and the surface roughness, it was clear that the surface durability of steel gears was improved by the increase in surface hardness due to the cavitation peening with little influence of surface roughness.