The microstructure, hardness and tensile properties of Mg-3Al, Mg-3Al-1.2Ca(AX31), Mg-3Al-1.2CaO(AO31), Mg-9Al, Mg-9Al-1.2Ca(AX91) and Mg-9Al-1.2CaO(AO91) alloys were investigated to identify the effect of CaO addition compared with Ca addition. The results show that the alloy element CaO has a good effect on high temperature tensile properties and CaO added alloys have superior high temperature performance compared with Ca added alloys. Addition of CaO results in the precipitation of Al2Ca phase (C15 phase) on the grain boundaries. This phase precipitated by the reduction of CaO leads to the refinement of a microstructure and improvement of tensile properties at elevated temperature similar to the Ca added Mg alloys. When added into molten Mg Ca makes solid solution in the Mg matrix up to 0.8wt %. But in case of CaO addition the thermally stable compound is precipitated directly without making solid solution with Mg matrix. Therefore, the high temperature tensile properties of CaO added Mg casting alloys are better than Ca added one at the same adding contents (1.2wt.%).