The effect of melt superheating treatment on the microstructure of Mg–1.5Si–1Zn alloy is investigated in this article. The microstructure of samples poured into sand and metal molds shows that the primary Mg2Si is considerably refined in size when the melt is superheated from 750 to 900 °C. However, its polyhedral morphology does not change. In addition, also the size of eutectic Mg2Si phase obviously decreases, but the morphology still exhibits Chinese script type. Meanwhile, it is concluded that the reduction of the heredity plays an important role in the grain refinement of Mg–1.5Si–1Zn alloy.