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Primary Mg2Si with a truncated octahedral morphology and size over a range of 8–15 μm were prepared by simultaneously adding Ca and Sb into the Al–Mg–Si melt. It is revealed for the first time that CaSb2 can act as the nucleus of primary Mg2Si during solidification, which changes the morphology and refines the size of primary Mg2Si significantly. It is useful for improving the mechanical properties of the Al–Mg–Si alloy. The skeleton-type growth processes of truncated octahedral primary Mg2Si crystals were identified by observation of the three-dimensional (3-D) morphologies of the crystals extracted from the Al–Mg–Si alloy. The content of Ca and Sb plays an important role in determining the morphologies of primary Mg2Si, which can be transformed from coarse dendrites to defective truncated octahedra and finally to the coexistence of perfect truncated octahedra and flat truncated octahedra with increasing the content from 0 to 0.2 and then to 0.5 wt%. Our study provided a one-step less-cost method to change the morphologies and sizes of primary Mg2Si, which is important to tailor new light-weight alloys with high strength and toughness. [ABSTRACT FROM AUTHOR]