The eutectic Si microstructure in Al-8.5wt.%Si alloy was changed from large flakes to fine lamellar when the Sc amount in the alloy reached 0.2 wt.%. 0.8wt.%Sc was optimal in terms of attaining the best modification effect. Study on the distribution of the modifiers and measurement of the surface tension of Al-8.5wt.%Si alloy melt with added Sr, Na, and Sc modifiers, respectively, reveals that Sc modifies eutectic Si by a decrease of surface tension, while Sr and Na modify eutectic Si mainly by an impurity-induced twinning mechanism. Al-8.5wt.%Si-0.4wt.%Sc alloy displayed approximately 50 and 70 % increases in tensile strength and elongation, respectively, over Al-8.5wt.%Si alloy in the cast state. It also presented approximately 65 and 70 % increases in tensile strength and elongation, respectively, over Al-8.5wt.%Si alloy at a ppt heat-treated state at 200℃ for 3 h.