The microstructures and mechanical properties of Mg-4.5Gd-2.6Nd-0.5Zn-0.5Zr new casting magnesium alloy were researched.It is found that the microstructure of the as-cast experimental alloy is composed of near equiaxed α-Mg, Mg12(Nd,Gd) and Mg3Gd phases. After optimizing the heat treatment parameters, the value of YS , UTS and A reach 205MP, 320MP and 4.0% at room temperature 145MPa,245MPa and 18.5% at 250℃.The strengthening principle of the alloy is studied in this paper. The fracture surfaces of Mg-4.5Gd-2.6Nd-0.5Zn-0.5Zr alloy was brittle fracture at room temperature tensile with and ductile fracture at 250℃ tensile with lots of dimples and tear ridges.