The effect of calcium addition on the microstructures of AZ91 magnesium alloy was investigated. It was found that a small amount of calcium in AZ91 alloy produced a large decrease in the α-Mg grain size and the dispersed fine β-Mg_ 17 Al_ 12 phases. In addition, some Al_4Ca particles were found to exist in the AZ91 alloy containing 0.5wt%Ca. EDS analysis and water-quenched technique revealed that the grain-refining mechanism of calcium for the AZ91 alloys was mainly attributed to the role of restricting growth of calcium in the primary α-Mg crystals. It was found that a small amount of calcium in AZ91 alloy produced a large decrease in the α-Mg grain size and the dispersed fine β-Mg 17 Al 12 phases. EDS analysis and water-quenched technique revealed that the grain-refining mechanism of calcium for the AZ91 alloys was mainly attributed to the role of restricting growth of calcium in the primary α-Mg crystals.