The effects of Ag addition on the microstructure and thermal stability of 6156 Al-Mg-Si-Cu alloy were investigated by means of hardness measurement, tensile tests, differential scanning calorimetry, and transmission electron microscopy. The results showed that addition of small amount of Ag to 6156 alloy did not change the precipitation sequence mainly beta '' and Q' strengthening phase but slightly accelerated the age-hardening rate and increased peak hardness at the same aging condition. The tensile properties enhanced about 30 MPa at the room temperature or thermal exposure at lower temperature (<100 degrees C). With the exposed temperature and time increased to 150 degrees C for 1000 h, almost no difference between the Ag-containing and Ag-free alloys. When exposed at 200 degrees C, the tensile strength of Ag-containing alloy became lower than that of Ag-free alloy because of the coarsening precipitations in matrix and boundary observed by TEM observed. For both alloys, thermal exposure at temperatures 100 degrees C for long time periods had no significant effect on tensile properties. Loss in strength was small and large with prolonging the exposure time at 150 and 200 degrees C, respectively.