Abstract: The effect of Zr content on the grain size of Mg-gGd-4Y alloys and the grain refinement mechanism of Zr were investigated. The results reveal that Zr can refine grains of the alloys greatly, and the grains become finer with the increase of Zr content. The microstructure analysis shows that there are at least one Zr-rich core in the almost each grain in the alloys with high Zr content, whereas the characteristic Zr-rich cores are not found in the alloy with low Zr content. It is suggested that the grain refinement mechanism of zirconium in the low Zr alloys be different from that in the high Zr alloys: the zirconium works mainly by restricting grain growth in the low Zr alloys, and by generating nucleants in the high Zr alloys.