Bolted flange connections with metal gasket have been used at higher pressure under higher temperature condition. Assembly procedures and tightening methods the connections including some types of metal gasket is empirically. Generally, it is known that the bolt preload which is required for satisfying an indicated leak rate in the connections including the metal gasket is not higher than that of the connections including sheet gaskets and spiral wound gaskets. However, no research for the evaluating the sealing performance in the bolted flange connections with metal gaskets has been conducted. In this paper, the leakage tests for the bolted flange connections with the metal flat gasket were conducted in the case where the maximum internal pressure of 7MPa and a bending moment are applied, where the leak rate is measured using the pressure drop method. Then, the sealing performance of the connections with the metal flat gasket was evaluated. In addition, using the FEM stress calculations, the flange stress distributions between the flange surface and the gasket were examined as the evaluation. As the result, it is found that the stress distribution at the contact surfaces between the metal flat gasket and the flange surface under the bending moment and the internal pressure. In the leakage tests, it was observed that the amount of the leakage (He gas) depends on the stress distributions and the plastic deformation of the gasket.