A self-consistent thermodynamic database of Al-rich Al-Si-Mg-Sc quaternary system was established by using CALPHAD method A solidification diagram for Sc-added A356 alloys was constructed for analyzing their solidification behavior The optimal additional amount of Sc in A356 alloys was predicted and experimentally confirmed to be 0.54 wt.% The grain refining mechanisms in Sc-supplemented A356 alloys were also proposed