In order to investigate the influence of tooth surface form on fatigue strength of steel gears, the case-hardened steel gears with non-ground tooth surfaces, ground tooth surfaces and crowned ground tooth surfaces were fatigue-tested using a gear testing machine. In the fatigue tests, the surface failure of the gear with a crown radius of 300 mm started to occur in the center of the tooth surfaces, and that of the gears with non-ground and ground tooth surfaces started to occur at the edge of the tooth surfaces. The fatigue strength using Hertzian contact stress of the gear with a crown radius of 300 mm was the highest, followed by those of the gear with ground tooth surfaces, the gear with a crown radius of 600 mm and the gear with non-ground tooth surfaces. On the other hand, the fatigue strength using normal load of the gear with ground tooth surfaces was the highest, and those of the other gears were almost the same. Therefore, it was found that the fatigue strength using Hertzian contact stress of the steel gears depends heavily on a crown radius of tooth surface, and that using normal load has little influence on it in this experimental range.