High-strength gears with high accuracy have been increasingly required for automotive engines and transmissions to make them compact in size and to attenuate gear noise. Nitriding has the merit of smaller dimensional change than carburizing but cannot provide sufficient fatigue strength. Thus, new nitrided steel has been studied to obtain sufficient fatigue strength. In addition, to make the gears more accurate, ion nitriding and gas nitriding have been tried.Consequently, a new hardening technique to provide compatibility of fatigue strength with accuracy was developed. The points of the technique are as follows: the use of 1.0% Cr-0.2% Mo-0.1%V steel in a state of bainite microstructure to increase the case depth and core hardness after nitriding results in a high fatigue strength equivalent to that of the carburized steel. Furthermore, selectively ion nitriding the gear teeth only results in a further limitation of dimensional change.