This paper studies the effect of Ce on the thermal stability of the Ω phase in an Al-Cu-Mg-Ag alloy by TEM and tensile testing. It has been shown that Ce substantially increases the nucleation density of the Ω phase by acting as the heterogeneous nucleation center. Most impor-tantly, Ce improves the thermal stability of the Ω phase by decreasing the diffusion velocity of Cu atoms and increasing the energy barrier of the thickening ledge nucleation, thus improving the strength of the Al-Cu-Mg-Ag alloy at both room- and elevated-temperatures.