It is important to improve the sealing performance of bolted flanged joints subjected to internal pressure. Recently, bolted joints with flexible flanges of stamped steel have been used in an oil-pan structure of automobile from a lightweight viewpoint. However, it is difficult to improve the sealing performance of flexible flanged bolted joints due to the reduction of the interface pressure. In this paper, a liquid sealant is applied to a box-shape flexible flanged bolted joints subjected to internal pressure in order to improve the sealing performance. In the experiments, a leakage is observed when a box-shape flanged joints in which liquid sealants are applied to the interfaces between aluminum flexible flanges with thickness of 1 and 3 mm and the steel (aluminum) flange with 10 mm thickness. Experiments are also conducted on bolted joints with asbestos gaskets. In addition, the effects of the adjacent bolt distance and bolt preload on the sealing performance are examined. The experiments are conducted on bolted joints in which similar thickness stamp steel flanges are joined. Moreover, the interface stress distribution of flexible bolted joints with liquid sealants is analyzed using finite-element method. Discussion is made on the sealing performance using the contact stress distributions at the interfaces. As a result, it is found that the sealing performance of box-shape bolted flanged joints with liquid sealant is more improved than that with the conventional gaskets.