The Mg-xGd-2.6Nd-0.5Zn-0.5Zr (x=0, 3.0, 4.5 and 6.0, wt.%) alloys were prepared by gravity casting and then T6 treatment. Microstructures of the alloy were observed using optical microscopy and scanning electron microscopy. Results show that the as-cast alloys contained network Mg3Gd phases, blocky and needle-like Mg12(Nd, Gd) phases. The α(Zr) particle inclusions in the α(Mg) matrix are also observed. Content of the secondary phases decreases as Gd content increases. Tensile test results show that the tensile and yield strengths of all the alloys increase as Gd content increases under the as-cast and T6 conditions, but the elongation exhibits the opposite trend. The blocky Mg12(Nd,Gd) phases appear and act as crack initiator and deteriorates the experimental alloys' ductility with the increase of Gd content, especially as Gd content increases to 6.0wt.%, so the Mg-6.0Gd-2.6Nd-0.5Zn-0.5Zr alloy has the lowest elongation value compared to the other alloys studied. After T6 treatment, the Mg-4.5Gd-2.6Nd-0.5Zn-0.5Zr alloy exhibits the optimal mechanical properties both at room temperature and 250 ºC.