This paper presents a study on hardened layer and bending fatigue strength of induction-hardened gears. The measurement of hardened layer and the bending fatigue test were carried out for induction hardened gears made of thermally refined, as-rolled and normalized S35C and S45C steels, and then the surface hardness, the hardened depth, the macro- and micro-structure of hardened layer and S-N curves were obtained. Effects of the pre-treatment and the tempering on the bending fatigue strength of induction hardened gears were examined.