The experimental data present that primary silicon is precipitated from eutectic and hypoeutectic Al–Si alloy melts. It attributes to solute redistribution on the chemical driving force. Si atoms are easy to segregate to form Si-Si clusters, resulting in primary silicon precipitated from hypereutectic Al–Si alloy. Another reason is that primary silicon is precipitated from solid-liquid interface front once solute concentration in the solidification front exceeds eutectic composition. Solute redistribution equations are derivate from Jackson-Chalmers equation. The third reason is that precious little impurity turn Heterogeneous Nucleation of Si atoms.