The effects of Ca addition on the microstructure and mechanical properties of AZ91 magnesium alloy have been investigated. The main changes in microstructure caused by Ca addition were the grain refinement in the as-east alloys and the formation of new Al2Ca phase along the grain boundaries. The tensile strength and elongation of the AZ91 magnesium alloy were improved by small addition of Ca due to grain refinement and replacement Of Mg17Al12 phase with new Al2Ca compound. However, when above 0.7wt.% Ca was added, mechanical properties of AZ91 magnesium alloy were decreased.