A cast Mg-5Sn-4Zn-2Al alloy was developed by adding 2%Al and 4%Zn (mass fraction) into a Mg-5Sn base alloy. The results showed that the combined addition of Al and Zn had more obvious effect on dendrite refinement than the single Al addition. The single Al addition remarkably increased the elongation from 6.6% to 22.4%. With further addition of Zn, Mg₃₂(Al, Zn)₄₉ phase was introduced into the solidified microstructure, and the yield strength and ultimate tensile strength were increased to 96 and 232 MPa while elongation was decreased to 14.8%. After aging treatment at 175 ℃ for 24 h, there appeared peak hardness of 83.5 HV by forming the rod-shaped MgZn₂ along c-axis and cubic Mg₂Sn precipitates into α-Mg matrix for Mg-5Sn-4Zn-2Al alloy. With the aging strengthening, the yield strength and ultimate tensile strength were further increased to 144 and 264 MPa, respectively. When the testing temperature was elevated to 150 ℃, the yield strength of peak-aged Mg-5Sn-4Zn-2Al alloy still attained to 138 MPa, which indicated that both the MgZn₂ and Mg₂Sn precipitates possess good thermal stability for elevated temperature property.