Rare earth (RE) elements have important effect on the microstructure and properties of cast aluminum alloys. With addition of 0.05, 0.15 and 0.25 wt% Er and Zr elements to the ZL205A alloy respectively, this article investigated the effect of different content of RE elements on the microstructure and properties of as-cast and T5 heat-treated alloys. The results show that adding micro scale Er and Zr elements can effectively improve the fluidity of liquid alloy, refine the grains and promote the theta phase to concentrate on the intersection of the grain boundaries. When the addition amount is 0.05 wt%, the mechanical properties of as-cast alloy reduced greatly while compared with the initial alloy; after T5 heat-treatment, the tensile strength, Vickers hardness and elongation can respectively reach 358 MPa, 107 and 2%, and the comprehensive performance is the best. T5 heat-treatment can make the microstructure and composition uniform, and also lead solute atoms to diffuse sufficiently, thus hindering the dislocation movement and sub-boundary migration during deformation process, finally taking adverse impact on the elongation.