The effect of trace elements Mg and Ag on the aging precipitation has been studied. It has been shown that both Mg and Ag have no obvious effect on the aging precipitates but influence the nucleation and growth velocities of the precipitates in binary Al-Cu alloy if added separately. However, when Al-Cu alloy contains both elements Mg and Ag, Mg and Ag atoms strongly attract each other and form atom clusters along the matrix {111} planes. These atom clusters help the Cu atoms aggregate and nucleate heterogeneously along the matrix {111} planes, which makes Mg atom clusters the optimized nucleation sites for Ω phase and inhibit the nucleation of θ′ phase.