Abstract: SEM and TEM were employed to study the structure stability of a new developed 718 alloy after aging at 680℃ for 1000 h. The results indicate that the main strengthening phase in a standard alloy 718 is γ″, while it is γ″ precipitated on increasing amount γ' in the new developed alloy and a compact morphology forms. The new developed alloy was proved to be very stable after long-time aging at 680℃. The morphology of phase on grain boundary in the standard alloy is long needle-like or lamellar while it is short rod-shaped or granular in the new developed alloy. The tensile properties of the two alloys at room temperature and 680℃ have a slight difference but the stress rupture and creep properties of the new developed alloy are greatly superior to the standard alloy. In comparison with the standard alloy, the new developed alloy has a better combination of mechanical properties and microstructure stability at 680℃.