In order to investigate the influence of shot peening on the surface durability of case-hardened steel gears, the case-hardened steel gears shot-peened with different shot velocities and shot diameters were fatigue-tested using a power circulating type gear testing machine. The hardness, the compressive residual stress and the surface roughness of the gears were increased by shot peening. The cause of failure of all the test gears was pitting due to surface cracking. The surface durability of the gear shot-peened under medium condition in this experimental range was higher than that of the non-peened gear. On the other hand, that of lightly or strongly shot-peened gear was just as high. Judging from the p_max-N curves and also the [A(σ_y/H_V)]_max-N curves, it is denoted in this experimental range that the surface durability of case-hardened steel gears was influenced by the hardness and the surface roughness of the gears. Therefore, it is proposed in this experimental range that the medium shot peening condition, that is, 60m/s shot velocity and 0.6mm shot diameter, should be selected in order to improve the surface durability of case-hardened steel gears.