The experimental alloy is designed and employed in high-performance industrial gas turbines as low-pressure turbine blades, working in temperature range of 750~900 ℃. The alloy contains high levels of refractory elements in order to increase the high-temperature mechanical properties. However, this can make the alloy prone to the formation of σ phase during service, which could deteriorate the properties further if the fraction of σ phase exceeds the safety allowances. In this study, the formation of σ phase during long-term thermal exposure, dissolution of the σ phase during rejuvenation process and their influence on stress-rupture properties of a hot-corrosion resistant nickel base superalloy have been investigated. During long-term thermal exposure at 800~900 ℃ for up to 1×10⁴ h, the σ phase formation is mainly in dendrite cores with a few at interdendritic regions. As the aging temperature increases, the precipitation rate of σ phase increases and the incubation time for nucleation of σ phase decreases. From the kinetic analysis, the σ phase form firstly in the vicinity or on the M₂₃C₆ in dendrite cores with the strong segregation of W, Cr and Co. The calculated activation energies of σ formation show that the early stage is related to Co and Cr diffusions and the steady stage is related to Mo diffusion. During solid solution process at 1000~1170 ℃, the σ phase precipitated during long-term thermal exposure dissolves to γ matrix. As the solid solution temperature is higher, the dissolution of σ phase becomes faster. Moreover, the σ phase does not embrittle the alloy. The reheat treatment of the alloy leads to the dissolution of precipitated σ phase and further prolongs the stress-rupture life efficiently.