Effects of rare earth (RE) on the morphologies of the recalescence interface, the growing primary Si during the solidification process and the structure after solidification of Al-80%Si alloy were investigated by means of high speed camera and SEM. The critical undercooling ?T₁ and ?T₂ for the morphology transition of the recalescence interface, the growing primary Si and the structure have been obtained. When the undercooling is lower than ?T₁, the morphology of the growing crystal during the solidification process is flake-like; and the structure after the solidification process is composed of large flake grains with pronounced edges and faces. When the undercooling is greater than ?T₂, the recalescence interface is a parallel one, and the structure after solidification is composed of homogenous and fine grains, and there exist several smooth spherical bulges on the surface of each grain. In the undercooling region from ?T₁ to ?T₂ , the recalescence interface and the growing crystal show dendritic features, but some of the dendrites are distributed regularly; after solidification, the structure is composed of refined equiaxed grains and flake grains. For Al-80%Si alloy, ?T₁ and ?T₂ are equal to 132 and 250 K, respectively. RE can reduce the values of ?T₁ and ?T₂ . When 1%RE is added into the alloy,?T₁ and ?T₂ are changed to 60 and 199 K, respectively.