Bolted flange connections with ring type joint gaskets have been used to seal the inner fluid under higher pressure and higher temperature conditions when soft gaskets cannot be used in the connections. However, few researches on the characteristics and the sealing performance evaluation for the bolted flange connections with metal ring gasket have been conducted. Therefore, assembly procedures and tightening methods of the connection with ring type joint gaskets is empirically and not fully elucidated. It is important to know the characteristics such as the contact gasket stress distribution which governs the sealing performance prediction, 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 differences of the sealing performance for the bolted flange connections (3″) by the re-use octagonal and oval type ring gaskets are examined. In the FEM calculations the contact gasket stress distributions and the range of plastic deformation are examined and the sealing mechanism is discussed. It is found that the basic sealing mechanism is caused by the plastic deformation at the contact area of the gasket. In addition, the leakage tests for the bolted flange connections with the nominal diameter of 20″ with the octagonal ring type joint gaskets being inserted are conducted, where the bolt tightening for the connection is carried out according to both ASME PCC-1 and JIS B2251, respectively. Using these results, the optimum tightening methods for the connection with ring type joint gaskets are discussed.