A homogeneous microstructure of as-cast magnesium alloys is desired to improve their mechanical properties when achieving lightweighting. Recently, it was demonstrated that the addition of SiC refines both Mg–Al and Mg–Zn alloys. The present work investigates the effect of SiC particle additions on the grain refinement of Mg–Zn alloys, including their addition amount, particle size, addition temperature and holding time. The microstructures were characterized using XRD, SEM and EDS. It was found that the addition of SiC particles refines the grains of Mg–Zn alloys. With increasing their amount and reducing the addition temperature and holding time, the grain size decreases. The optimal SiC particle size for nucleation of alpha-Mg was found to be around 2 µm. The responsible refinement mechanism is attributed to the formation of (Mn, Si)-enriched intermetallics by the interactions between SiC and impurity Mn in alloys.