The complex modification of primary Mg2Si in an Al–20Mg2Si alloy by simultaneous addition of yttrium (Y) and antimony (Sb) was investigated in the present work. It was found that the combined addition of 0.5 wt% Y–Sb had a more significant modifying effect than the single addition of an equivalent amount of Y or Sb. After modification, coarse Mg2Si dendrites were changed into fine polyhedra with an average size decreasing from 86 to less than 18 μm. It has been demonstrated that Sb participated in the formation of Mg3Sb2, which acts as a heterogeneous nucleus of primary Mg2Si and hence significantly refines their sizes. Meanwhile, Y could obviously adsorb and poison the preferred growth along the directions of primary Mg2Si crystals, and hence not only changed their final morphology to a truncated octahedron but also reduced their sizes. Furthermore, the skeleton-type growth process of the truncated octahedral primary Mg2Si in the Al–20Mg2Si alloy co-modified by 0.5 wt% Y–Sb was also revealed. Our study provides a new insight into the design of more efficient modifiers by the combined addition of a refiner (e.g. Sb) and a growth inhibitor (e.g. Y), which is critical to tailor new light-weight alloys with high strength and toughness.