The influences of Sc contents on the microstructures of Al-5.5Mg-0.5Mn-XSc-0.1Zr (0.05≤X≤0.50) as cast alloy and the morphologies of the second Al3(Sc,Zr) precipitates as well as their strengthening effect after aging treatment were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and micro-hardness tester. The results show that when X≤0.09, no precipitates containing Sc are found in as cast alloy. On this case, the alloy possesses the typical columnar dendritic microstructure, and the strengthening effect after the aging treatment is weak. While for the alloy with 0.16%~0.23% Sc, primary and eutectic Al3(Sc,Zr) particles precipitate during solidification, which not only refine grains, but also not impair the thermal stability of the second Al3(Sc,Zr) precipitates, and the strengthening effect is strong. When X≥0.23, a large volume of primary and eutectic Al3(Sc,Zr) particles precipitate. Though this can refine the grains strongly, but a side effect is that the Zr content in the alloys after solidification would be severely decreased, resulting in the second Al3(Sc,Zr) precipitates becoming less thermal stable. After aging at 450 ℃ for 24 h, the second Al3(Sc,Zr) precipitates coarse a lot, and its strengthening effect becomes very weak