The addition of SiC particles effectively refined the grains of a range of aluminum-free Mg-Zn alloys. Simultaneously, the corresponding mechanism of grain refinement has also been discussed. In the present work the effect of SiC on the microstructure of as-cast Mg-Zn alloy was investigated using optical and scanning electron microscopy. The results show that the primary alloying element Zn segregates at the grain boundaries and the phase Mg₇Zn₃ locates inside Mg matrix of Mg-Zn grains. The efficiency of SiC as a grain refiner can be seen with 45% average grain size reduction of Mg-3Zn alloy at 0.3 wt% SiC addition, indicating an existence of close relationship between the contents of SiC and Zn to affect the grain size. The interaction between SiC and the impurities such as Mn and Fe in Mg-Zn alloys is suggested to explain the grain refinement.