Bolted joints are used in many industrial products such as mechanical structures, automobiles, airplanes, chemical plants, and so on. In designing bolted joints, the bolt preload in the joints should be determined appropriately and rationally for preventing accidents due to bolt loosening and fracture. Also, in actual bolt tightening, it is important that the bolts should be fastened with a designed bolt preload range. Several methods for tightening bolts are well known, one of which is the torque clamping method. In this method, the designed bolt preload can be obtained by a controlled tightening torque. The relationship between the tightening torque and bolt preload is assumed to be linear in the elastic range, and the bolt preload is often calculated from the tightening torque using the nut factor. However, in some cases, where the relationship between the tightening torque and the bolt preload is not linear. One of the reasons is the effect of bearing surface angle at bolt head. In this study, the effect of the bearing surface angle at the bolt head on the nut factor is examined experimentally. Also, it is known that the bearing surface angle at the bolt head affects the contact stress at the bearing surface of the joint member. The contact area ratio is newly proposed and obtained. As a result, a method to obtain an appropriate bearing surface angle at the bolt head and tightening torque range is proposed, taking the effect of contact area ratio factor into account.