Microstructural variation is quantitatively characterized by measured undercooling. For near-eutectic Al-13wt%Si alloys, microstructure transitions, which include a phase change from eutectic to primary alphaAl dendrite plus eutectic and a morphological change from alpha-Al dendrite to equiaxed Al grains, were observed with increasing undercooling levels. For hypereutectic Al-Si alloys, microstructure transitions, which include a morphological change of primary and eutectic Si crystals, were also obtained quantitatively by increasing undercooling.