The solidification diagrams of A357-Sc, A360-Sc and A380-Sc were constructed on the basis of the Scheil-Gulliver solidification simulations and previously well-established thermodynamic database, from which the optimal additional amounts of Sc in Al-Si-(Mg) series hypoeutectic alloys were designed for the best mechanical properties, and then validated by the microstructure observation and mechanical property test. With the designed levels of Sc addition, the eutectic (Si) particles were well refined and the primary (Al) grains evolved from the elongated ones to the equiaxial ones. The primary/eutectic (Al)/AlSc2Si2 interfaces were determined using the high resolution transmission electron microscope technique to be partially coherent in three dimensions. Furthermore, it was finally understood that the grain refinement of eutectic (Si) and the occurrence of partially coherent AlSc2Si2 phase with (Al) matrix due to the addition of Sc allow the high density of dislocations to accommodate at grain boundary, and thus improve the mechanical strengths and ductility of Al-Si-(Mg) cast alloys.