Abstract: The microstructure and mechanical properties of an Al-Cu-Mg-Ag alloy aged at 165℃ were investigated, and the underage (165℃ × 2 h) samples were subjected to thermal exposure at 150, 200, 250 and 300℃ for 0 to 1000 h. The results indicate that at the exposure temperature of 150℃, the residual strength of underage samples appears to increase first and then decrease, and the peaking strength is obtained when lasting for 100 h. The change in elongation of the samples has the same tendency as the change in residual strength. Compared with underage condition, the mechanical properties of the samples have no obvious variation after the 150℃/1000 h exposure, and thus the alloy represents a superior heat-resistance ability. With prolonging exposure time and increasing temperature, the residual strength of the samples appears to decrease and the elongation appears to increase at 200, 250, and 300℃. The residual strength of the samples obviously decrease at the exposure temperature of 300℃, and the tensile strength of the samples after the 10 h exposure is 272.5 MPa decreasing to 114.5 MPa after the 100 h exposure. At all examined temperatures the Ω plate thickening shows a linear dependence on time. With increasing exposure temperature, the Ω plate thickening kinetics greatly increases and the precipitation free zones broaden in grain boundary.