Bolted connections with gasket such as box-shaped flange connections have been widely used in mechanical structures, such as nuclear and chemical industry, and so on. They are usually used under internal pressure. The high sealing performance of flange connection is required to avoid the unexpected leakage accident. The two methods, which are Diagonal and Clock-wised, are used in the bolts tightening procedure for the pipe flange connection. The effects of the difference between Diagonal and Clock-wised method on the sealing performance of pipe flange connection have been examined. In this paper, the contact gasket stresses of these connections under internal pressure are analyzed using the finite element method (FEM) of each bolting procedure, taking into account a hysteresis in the stress-displacement curve of a gasket. And then, using the contact gasket stress distributions obtained from FE analysis and the relationship between gasket stress and leak rate obtained from a gasket sealing test, a method for estimating an amount of leakage is examined. The leakage tests were also conducted to measure an amount of gas leakage using an actual box-shape flange connection with the gasket. In addition, the effect of bolt spacing is examined on the sealing performances of box-shape flange connections under internal pressure. The effects of the above factors are discussed on the sealing performance in designing box-shape flange connections.