Pipe flange connections with gaskets have been widely used in mechanical structures such as nuclear plants, petroleum refining plants and chemical plants and so on. Those connections are usually exposed to elevated temperature with internal pressure. Thus some investigations on the mechanical behavior of pipe flange connections with gaskets under thermal condition such as the sealing performance, the contact stress at the interfaces and a variation in the axial bolt force, have been performed. However, it is important to investigate the sealing performance of pipe connections under long term thermal exposure for using them safely. In this study, the effects of cyclic thermal condition on the sealing performance and mechanical behavior in pipe flange connection will be discussed based on the experimental leakage tests of pipe flange connections with gasket and FEM analyses.