Abstract: The effect of electro-magnetic stirring process parameters on the microstructure of semi-solid AZ91D alloy was studied. The results showed that the shape of solid particles in the alloy was rosette-like or granular. With the stirring power increasing, the rosette-like particles disappeared gradually, and solid particles got more and more spherical. A higher power made the solid particles of the alloy more spherical, because the main evolution mechanism of microstructure of the alloy was the dendritic arm melting method. A higher frequency introduced a skin effect on the stirred semi-solid slurry, which made a temperature difference between the edge and the center. The slurry under electro-magnetic stirring took a secondary flow besides the horizontal rotating flow, which made the slurry move from the edge to the center in cycles. This secondary flow resulted in a promotion of dendritic arm being melted away from the dendrite grains by the large temperature fluctuation.