In automotive components, there is a significant weight saving potential in converting to magnesium alloys. Adding Ca element is one of the most effective ways to improve the temperature and mechanical properties of magnesium alloy. However, as hot-cracks often appear in such materials, Ca addition will thus encumber the development of this magnesium alloy. There are few reports on the hot-crack mechanism of magnesium alloy with Ca. This paper focused on the investigations as why to results in the hot-crack phenomenon of Ca addition. These results indicated that Ca addition affects the solidification process of AZ91D alloy, such as elevating the tendency of divorce eutectic and forming the new temperature of Al2Ca phase. In addition, the Ca-contained phase distributed as the net-shape forms on grain boundary and results in lower boundary strength of the liquid film. Both of the above reasons would deteriorate filling capacity of the melt and the hot-crack resistance during solidification stage, resulting in the worst hot-crack property of this alloy. (C) 2005 Springer Science + Business Media, Inc.