Microstructural changes with varying amounts of Ca in cast AZ31-xCa (x: 0.7 wt.%, 2.0 wt.%, 5.0 wt.%) alloys were investigated. According to transmission electron microscopy (TEM) analyses, it was experimentally confirmed that the C36-(Mg,Al)2Ca phase with a di-hexagonal structure formed at interdendritic regions in as-cast AZ31 alloys with no more than 2 wt.% Ca. On the other hand, as the Ca content exceeded 2 wt.%, the lamellar structure consisting of the α-Mg phase and the Mg2Ca phase with a C14 structure formed at interdendritic regions instead of C36 phase. Plate-like Al2Ca precipitates with a C15 structure also formed on the basal plane inside the α-Mg grains.