The effects of trace amounts of Ca on the microstructure and mechanical properties of as-cast Mg-Sm- Gd-Zn-Zr alloy were systematically investigated mainly by means of transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and tensile test. The results reveal that the dominant intermetallic compound in Mg-Sm-Gd-Zn-Zr alloy is Mg₃RE phase with significant enrichment of Zn. The addition of Ca in Mg-Sm-Gd-Zn-Zr alloy can not only refine grains, but also promote the formation of Mg₅RE phase and solute-enriched stacking faults (SESFs). The coherent interface indicates that Mg₃RE phase can act as the nucleation site of Mg₅RE phase. Finally, the decreased grain size and the formation of Mg₅RE phase and SESFs in the Ca-modified alloy are beneficial to the enhancement of mechanical properties.