The effect of Mg and Zn additions on the microstructure evolution and mechanical properties of an Al-Cu-Li alloy was studied by transmission electron microscopy and tensile testing. Results show that additions of Mg and Zn can accelerate the precipitation of δ′ phase nucleation and inhibit its growth, so the δ′ phase becomes finer and more dispersive. In the under-aging condition, a lot of GP zones are formed in the alloy containing Mg and Zn. The alloy only containing Mg also has many GP zones. But no GP zone is found in the alloy only containing Zn, while only a few coarse θ′ is found. The above results reveal that Mg promotes the formation of GP zones while the single role of Zn is not significant; Zn has a better effect when it acts with Mg together. As the aging proceeding, GP zones in the alloy containing Mg and Zn transforms into θ′/θ′′ phase and plenty of θ′/θ′′ phases are formed in the alloy only containing Mg, but θ′ phase dose not change obviously in the alloy only containing Zn. It indicates that Mg is the key element to accelerate θ′/θ′′ phase formation, and Zn can promote the formation only with Mg.