The contact gasket stress distribution and the hub stress of pipe flange connection under external bending moment and internal pressure are analyzed using three dimensional elasto-plastic FEM. The effect of external bending moment on the sealing performance is evaluated and the amount of the leakage is predicted using the contact gasket stress distribution. From the obtained FE analyses, it is found that a large decrease in the contact gasket stress at the tension side occurs and a small increase at the compression side occurs. The difference in the change in the contact gasket stress occurs due to the non-linear hysteresis characteristics of stress-contract curve of the gasket (spiral wound gasket) used in the study. The FE analyses also suggest that the loading order, that is, the following two cases are analyzed, 1) an internal pressure is applied at first and then the external bending moment is applied to the connection, 2) the external bending moment is applied to the connection and then the internal pressure is applied, also affects the sealing performance due to the non-linear contract characteristic of the gasket. The sealing performance when the external bending moment is applied prior to the internal pressure is degraded more than that when the internal pressure is applied prior to the external bending moment. The leakage tests using helium (He) gas and the measurement of the hub stress were carried out, and then the amount of the leakage was measured. The amount of leakage from the connection is predicted using the contact gasket stress distribution obtained from FE analyses and the results are in a fairly good agreement with the measured results. It is found that the effect of the external bending moment is substantial on the sealing performance of the connections. Also, the calculated hub stresses are fairly consistent with the measured results.