Metal gaskets have been used in bolted flanged connections under higher pressure and higher temperature conditions. However, it is difficult to estimate the sealing performance of the metal contact interfaces in the connections. Thus, it is necessary to study on the sealing characteristics of the metal to metal contact gasket in the connections under higher internal pressure and higher temperature. In this study, firstly, the sealing behavior test taking into account load history is carried out in detail using metal flat gaskets according to JIS B2490. Secondly, to examine the effect of the gasket contact area and surface roughness of metal gasket on the sealing behavior of metal-metal contact, the sealing behavior test is carried out using metal flat gaskets which has several types of gasket width and surface roughness. Thirdly, the amount of leakage is measured for bolted flanged connections with metal flat gaskets. The relationship between the gasket contact stress and the leak rate is obtained. Furthermore, the contact gasket stress distributions in the connection are analyzed using FEM stress analysis. Using the obtained gasket stress distribution, the sealing performance is estimated taking into account the gasket contact area and surface roughness of the metal flat gasket. The estimated result is compared with the measured results. In the results, the effect of the yield stress and the plastic deformation of the gasket surface are found to be important on the sealing mechanism.