Since scatter in axial bolt force exists in assembling pipe flange connections, it is important to evaluate the effect of scatter in axial bolt force on the sealing performance of pipe flange connection. The FE and experimental analyses were done to evaluate the effect of scatter in axial bolt force on the sealing performance of pipe flange connections with 3″ (NPS 3″) and 20″ (NPS 20″) nominal diameters. The axial bolt force was measured with strain gauges mounted on the shank of each bolt. The two kinds of torque controlled assembly procedures, ASME PCC-1 procedure and JIS 2251 procedure, were evaluated as an assembling procedure and compared the scatter in axial bolt force with that of the uniform axial bolt force condition. The results revealed that the significant difference was not observed between ASME PCC-1 and JIS 2251 procedures. The scatter in axial bolt force were not changed at elevated temperature, however, the axial bolt force increased when increasing the temperature. Therefore, the tightness of the flanged connection is improved with increasing the temperature up to 200 °C. The assembly efficiency was evaluated with the experimental results of 20″ nominal diameter leakage tests and FE analyses. In addition to the scatter in the assembly axial bolt force, the flange rotation affected the axial bolt force when internally pressurized. As a result, the tightness parameter, Tp, was slightly lower than that evaluated with the assembly efficiency, η=0.85, defined in ASME PCC-1 procedure. The results are discussed and a modified assembly efficiency is proposed.