The purpose of this study is twofold: to clarify the factors governing the torsional strength of surface induction hardened parts and, to present a method for strengthening automotive shaft parts for their weight reduction. The torsional strength against Mode III fracture can be expresssed by a new indicator, “equivalent hardness” defined as an average hardness weighted with the radius squared. If the equivalent hardness is continuously increased, the fracture mode change from Mode III to Mode I. The torsional strength against Mode I fracture is governed by grain boundary strength. Accordingly, the key-points in increasing the torsional strength of surface induction hardened parts are to raise the equivalent hardness and increase the grain boundary strength of the steel. By application of this method, the torsional strength of steel can be raised by 50%, which, in turn, enables about a 25% weight reduction for shaft parts.Over recent years, product and material engineers have pursued automotive component mass reduction by increasing material strength. An increase in the torsional strength of shafting has been a subject of particular interest. As the main function of a shaft is the transmission of torque, the main property required of them is high torsional strength. Shafts are normally manufactured by induction hardening a medium-carbon streel. Although many have reported on the bending strength of induction hardened parts (1, 2, 3 and 4), only a few have reported on the static strength and the strength of parts under repeated stress in connection with the torsional strength of induction hardened parts (5, 6). Therefore, the factors governing the torsional strength of surface induction hardened parts are not clear.The purpose of this research is to clarify the factors governing the torsional strength of induction hardened parts and to devise a method for strengthening automotive shafts in order to reduce their mass. As static torsional strength is the most important property of automotive shafts, our research focused on the static torsional strength of surface induction hardened parts. Since induction hardened parts normally have a hardness distribution from surface to center, torsional strength is thought to be determined by a combination of hardness distribution and stress distribution. Therefore, we first investigated the effect of hardness distribution on the torsional strength of induction hardened parts. We then examined, from the viewpoint of grain boundary strength, the factors governing the torsional strength related to Mode I fracture which is often encountered in strengthening steels.