Sealing of contained fluids is the primary performance required for pipe flange connections. It is well known that the leakage is likely to occur when contained fluids at high temperature are concerned. Due to the differential thermal expansion of each part, bolt preloads that tighten a pair of pipe flanges tend to decrease. Therefore, it is significantly important for the joint safety to estimate the amount of bolt preload reduction at the design stage. In this paper, a finite element approach is proposed to analyze thermal and mechanical behavior of pipe flange connections at elevated temperature, by incorporating the stress–strain curves of sheet gaskets measured in the temperature range which covers its usual service condition. Then, the reduction rate of bolt preloads at elevated temperature is systematically evaluated. The analytical objects are pipe flange connections tightened with aramid sheet gaskets. When a pipe flange connection is subjected to thermal load, the gasket stress usually decreases along unloading curves. The temperature dependency of gasket stiffness is considered by defining Young’s modulus in unloading E*, which can be introduced into FE formulation using ordinary solid elements. Numerical results show that bolt preloads are decreased by as much as 30% of the initial value when using aramid sheet gaskets of 3 mm thickness. The effectiveness of the proposed numerical method has been confirmed by comparing the numerical results of bolt preload reduction to experimental ones.