Bolted flange connections with ring joint gaskets have been used to seal the inner fluid under higher pressure and higher temperature condition when soft gasket cannot be used in the connection. The ring joint gaskets are assumed that they can seal inner fluid by the metal-metal contact between the ring joint gasket and the grooves of flanges under higher pressure condition. However, no research on the characteristics and sealing performance evaluation for the bolted flange connections with metal ring gasket has been conducted. Therefore, assembly procedures and tightening methods of the connection with ring joint gaskets is empirically. It is important to know the characteristics such as the contact gasket stress distribution, the sealing performance and the sealing mechanism in the bolted flange connections for determining optimum tightening methods and the bolt preload. In this paper, the leakage tests for the bolted flange connections (3") with the octagonal ring joint gaskets are conducted. Measuring the leak rate of inner fluid from the connections using the pressure drop method, the sealing performance of the connections with the ring joint gaskets is evaluated. In addition, using the 3-D FEM stress calculations, the gasket contact stress distributions for the connections with the octagonal ring joint gaskets are examined. Using these results, the optimum bolt preload of the connection with octagonal ring joint gaskets are discussed. Furthermore, the leakage tests for the bolted flange connections (20") with the octagonal ring joint gaskets are conducted in the case where the bolt tightening is carried out according to both ASME PCC-1 and JIS B 2251, respectively. The optimum tightening methods for the connection with octagonal ring joint gaskets are discussed.