Since an external bending moment affected the sealing performance of pipe flange connection, it was important to investigate this effect. This paper analyzed the contact gasket stress distribution of pipe flange connections and evaluated the effect of external bending moment on the sealing performance from the viewpoint of changes in contact gasket stress. The study included the FE analyses and the experimental leakage tests. The FE analyses revealed the large decrease of contact gasket stress at tension side and small increase at compression side. The difference in change in contact gasket stress was caused by the non-linear hysterics characteristics of stress-displacement curve of gasket. The FE analyses also revealed that the loading order, internal pressure and external bending moment, also affected the sealing performance due to the non-linear deformation characteristic of the gasket. The experimental leakage tests using helium (He) gas were analyzed by the finite element method and discussed. This paper also evaluated the stress distribution in the pipe flange under external bending moment. The results suggested that the hub stress dominated the flange structure and the most important factor in designing the flange.