Abstract: SEM was used to observe the changes of the microstructure in hypereutectic Al-18wt%Si-Mg casting alloys with various rare earth additions.The effect of rare earth on the crystallizing behavior of Mg2Si phase and mechanical properties were discussed.Mg2Si phase nucleated on the surface of the eutectic Si flakes in the un-fully modified alloys,growing as net or skeleton shape,however,very few and fine Mg2Si particles were isolated at the boundaries of eutectic cells in the fully modified alloys.Rare earth had an important influence on the crystallizing behavior of Mg2Si phase in Al-Si-Mg casting alloys and their mechanical properties.This thought to be related to the amount of the modification of α (Si ) and eutectic Si phases.