Optical microscope and SEM were used to observe the changes of the microstructure of Al 11.6% Si 0.4% Mg alloys with varying strontium additions and the effect of strontium on the crystallization of Mg 2Si phase was discussed. It is found that Mg 2Si phase nucleates on the surfaces of the eutectic silicon flakes in the unfully modified alloys, growing as meshwork or bamboo shoot shape, however, very few and fine Mg 2Si particles phase are isolated at the boundaries of the eutectic cells in the fully modified alloys. Strontium has an important influence on the crystallization of Mg 2Si phase in Al Si Mg casting alloys and it is thought to be related to the increase of the amount of dendritic α phase and the modifying degree of eutectic silicon phase.