Pipe flange connections have been used in chemical plants, petro-chemical industry and so on. They are tightened with a lot of bolts and nuts by torque wrench, impact wrench and so on. When the impact wrench is used for tightening a lot of bolts and nuts in pipe flange connection with gaskets, it is well known that a large scatter is assumed to occur in tightening the lots of bolts in the connection using the impact wrench. In the present paper, the scatter in bolt preloads in a pipe flange connection with 20" nominal diameter using impact wrenches was measured. The ratio of the maximum and the minimum bolt force is called the tightening coefficient. The tightening coefficient is calculated taking into account the interference of the adjacent bolts. The value of the coefficient is compared with the measured results obtained from torque wrench tightening. Then, FEM calculation for analyzing the gasket stress distribution in the process of bolt tightening is carried out. Using the obtained gasket stress distribution and the data obtained from the fundamental gasket characteristics, an amount of leakage is estimated. In addition, the bubble leak testing was carried out. The estimated result is in a fairly good agreement with the measured result. Discussion is made on the tightening coefficient by impact wrenches and the effect of the scatter in bolt preloads on the sealing performance.