The grain refining process of an AZ91D Mg alloy by Sr addition was studied and the heterogeneous nucleating particles of α-Mg were investigated by electron probe microanalysis (EPMA). With 0.6 wt% Sr addition, the mean grain size of AZ91D alloy was refined from 235.4 μm to 52.5 μtm at the one-half radius of the ingot. The morphology of primary crystal changed from a sixford symmetrical shape to a petallike shape. Mg-Sr-Al-Fe-Mn heterogeneous nucleating particles were observed at the grain centers and Sr solute atoms presented segregation along the grain boundaries. Grain refinement was facilitated by both the Mg-Sr-Al-Fe-Mn nucleating particles and the Sr solute atoms, and the former played a dominate role in the process.