Cast defects are common in cast alloys and they are difficult to eliminate without deformation. They strongly degrade the mechanical properties of cast alloys. The addition of some elements can affect the number of cast defects. In this work, the deleterious effect of Sn addition on the mechanical properties of Al-Si alloys has been investigated via 3D-computed tomography, SEM and TEM. Amorphous Sn oxides were found near the alumina film or formed enclosures with alumina film. The melt containing high Sn content was trapped by enclosures, causing more shrinkage pores during solidification. Cracks likely initiated and expanded along these pores and brittle amorphous Sn oxides, deteriorating the mechanical properties. This work suggests not adding Sn to various Al alloys when used in a cast state.