Due to high specific strength and low density, AZ series magnesium alloys have been receiving high interest as a lightweight material. However, their industrial application is limited due to the phenomenon that the strength decreases at elevated temperature by the occurrence of softening effect because of the Mg₁₇Al₁₂ phase decomposition. To solve this problem, many research were conducted to increase the high-temperature strength by forming a thermal stable second-phase component by adding new elements to the AZ magnesium. Especially, adding Ca to AZ magnesium has been reported that Ca forms the new second-phase. However, studies about the analysis of decomposition or precipitation temperature, formation composition, and components to understand the formation behavior of these precipitated phases are still insufficient. Therefore, the effect of Ca addition to AZ61 on the phase change and microstructure of the alloy during annealing was investigated. As a result of analysis of the initial and heat-treated specimen, AZ61 formed α-Mg matrix and precipitated phase of Mg₁₇Al₁₂, and AZX611 formed one more type of precipitated phase, Al₂Ca. Also, Al₂Ca was thermal stable at high temperatures. And after annealing, the laves phase was decomposed to under 10 μm size and distributed in matrix. (Received February 3, 2021; Revised February 10, 2021; Accepted March 17, 2021)