The improvements of microstructures and properties of a high strength aluminum cast alloy were studied. The effects of rare earth elements on the microstructures and mechanical properties of the high strength cast alloy Al-Cu-Mg-Si were investigated. The result shows that the addition of rare earth elements can change the microstructures in refining the grain size of the alloy and making the needle-like and laminar eutectic Si to a granular Si. With the increase of the rare earth, the tensile strength and elongation of the alloy increase first and then fall down. The mechanical properties of the alloy will reach the highest value when the content of rare earth elements is about 0.7%. The improvements of microstructures and properties of a high strength aluminum cast alloy were studied. The effects of rare earth elements on the microstructures and mechanical properties of the high strength cast alloy Al-Cu-Mg-Si were investigated. of rare earth elements can change the microstructures in refining the grain size of the alloy and making the needle-like and laminar eutectic Si to a granular Si. With the increase of the rare earth, the tensile strength and elongation of the alloy increase first and then fall down. The mechanical properties of the alloy will reach the highest value when the content of rare earth elements is about 0.7%.