Bolted pipe flange connections have been widely used in mechanical structures such as oil, chemical plants and so on. Sometimes leakage accidents occur from the connections. In assembling the pipe flange connections, the bolt preloads are scattered and then the connections are subjected to external bending moment due to earthquake as well as internal pressure. In the present paper, the changes in the axial bolt forces are measured in assembling. The bolts are tightened with torque wrench. Using the history of the bolt force changes, the gasket stress distributions on the connection with gaskets are analyzed using FEM. Then the gasket stress distributions on the connections under external bending moment and internal pressure are analyzed. Using the gasket stress distributions, the amount of leakage of the connections is estimated while the basic gasket characteristics have been measured according to JIS B 2251. The amount of leakage of the connection was measured for the various gasket stresses. The estimated results are in a fairly good agreement with the measured results. It is found that the effect of the scatter of bolt preload is substantial on the sealing performance of the connections. Discussion is made on the sealing characteristics of the connections. The change of axial bolt force obtained from experimental result and FEM result are fairly good agreement.