Generally, bolts and nuts in bolted pipe flange assembly have been tightened using torque wrenches. On the other hand, recently, impact wrenches have been applied for tightening assemblies from working efficiency stand point In the use of the impact wrenches, since the value of tightening coefficient is bigger, it is necessary to take account the value of tightening coefficient when bolts and nuts in important bolted joints are tightened. In the present study, tightening method of nuts and bolts in bolted pipe flange joints is examined when impact wrench is used from FEM calculations and experiments. In this study, with an electric impact wrench, we performed the tightening experiment of the comparatively small size pipe flange, and FEM analysis was carried out for examining the axial tension characteristic. As a result, it is demonstrated that the scatter in bolt tightening forces using impact wrenches can be controlled as the same as that tightened using torque wrenches when the impact wrenches used is selected adequately and their tightening method is correct.