Mechanical properties and microstructural evolution of an Al-Cu-Mg-Ag alloy during thermal exposure at elevated temperature were characterized by means of tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation. The tensile test results suggested that tensile strength (σb) and yield strength (σs) reduced with both increasing exposure time and temperature. When exposed at 150°C, the Al-4.87Cu-1.2Mg-1.0Ag alloy exhibits good structural stability due to the excellent thermal stability of the Ω phase. SEM results showed that the specimen was fractured in a combined manner of intergranular and transgranular fracture. TEM results suggested the reduction of mechanical properties of samples was attributed to the coarsening of Ω phase and the precipitation of S' phase during thermal exposure.