In order to simulate the precipitation effect during the aluminum metallization in the VLSI processing, an analytical model was constructed in this work. The nucleation behaviors of the silicon and copper precipitates within the aluminum - 1.0 weight percent silicon - 0.5 weight percent copper were studied. The volume free energies and interfacial energies were estimated, and the activation energy barriers for the precipitate formation were calculated. Silicon precipitate is more likely to nucleate than copper precipitate due to its lower interfacial energy and strain energy. The mechanical and electrical properties of the aluminum interconnects can be improved by the precipitate hardening.