[en] The effect of Barium (Ba) addition (0, 0.05, 0.15, 0.5 wt. pct) on the microstructure and mechanical properties of an Al-7Si alloy was studied. Differential scanning calorimetry was employed to analyze the nucleation behavior of eutectic Si. Transmission electron microscopy was used to clarify growth mechanism of eutectic Si. The result indicates that the addition of 0.15 wt. pct Ba modifies the morphology of eutectic Si from coarse flake-like to fine fibrous, which dramatically improves the ultimate tensile strength and elongation. However, Ba addition (≥ 0.15 wt. pct) induces the formation of coarse blocky Ba containing phase (i.e. Al₂Si₂Ba phase) and porosity increment, and resultantly has a deleterious effect on the ductility. Thermal analysis reveals that Ba addition suppresses the nucleation of eutectic Si due to the formation of Ba₃P₂ phase. Significant multiply Si twins with high density was observed in the Ba containing alloy, which provides an experimentally support to the impurity-induced twinning growth mechanism.