The authors have been evaluating the effects of the material properties of spiral wound gaskets (SWG) and the sealing performance of the pipe flange connections at elevated temperature with numerical and experimental analysis. The contact gasket stress increased by the mismatch of the thermal expansion between the gasket and flange/bolt as increasing the temperature and the sealing performance of the pipe flange connection was improved. The authors have also evaluated the effect of scatter in axial bolt force and revealed the assembly efficiency is not as high as expected in case of large nominal diameter pipe flange connection. Following the previous studies, the sealing performance of pipe flange connection and the assembly efficiency with SWG at elevated temperature were further evaluated with the experimental leakage test and 3-D FE analysis in this study. In order to make a precise evaluation, the experimentally obtained material properties of SWG were used in the analysis. The uniform axial bolt force tests and the scattered axial bolt force test were done to compare the sealing performance and the assembly efficiency evaluated by the ratio of the tightness parameter of each test at elevated temperature. Although the leak rate improved as increasing the temperature, the assembly efficiency decreased. The leakage based assembly efficiency was proposed according to the experimental results and numerical analyses.