In order to clarify the performance of 625 alloy during 10000 h long term thermal aging at 750℃, the mechanical behavior and microstructure evolution were investigated by means of Scanning electron microscopy (SEM), transmission electron microscopy (TEM), as well as tensile and hardness tests. Results show that a phase transition from the metastable γ" phase to the steady-state δ phase occurs in 625 alloy at 750℃. At the initial stage γ" phase rapidly precipitates and grows in matrix, and then decreases and disappears with the extension of thermal aging. While the transformed δ phase gradually nucleates and grows in size, and turns to be the main strengthening phase in 625 alloy in the long run. With the prolonging of the thermal aging time, tensile strength, yield strength and hardness of the alloy are continuously improved, with the elongation constantly reducing. After 3000 h’s thermal aging treatment, the mechanical properties tend to keep stable. There was no significant difference between the two types of δ phase in the reinforcement of 625 alloy.