Influences of the three melt treatment processes (namely, refinement with carbon inoculation, electromagnetic stirring and refinement & electromagnetic stirring processing) on the microstructures of AZ91D alloy were explored experimentally. The results indicate that the micron carbon powders inoculation processing with 0.1% (mass fraction) addition level makes the grain size of the primary phase (α-Mg) decrease to approximate 40% that of the initial primary crystal, from about 530 μm to 200 μm around.The electromagnetic stirring processing not only decreases the grain sizes sharply, but also transfers the grain shape of α-Mg from coarse dendritic to nearly spheralitic with the shape factor from about 0.1 to approximate 0.8. And the refinement & electromagnetic stirring processing improves grain shape of α-Mg further and refines grain size to about 72 μm, less than 15% that of the initial primary crystal.