The microstructure, phase composition and properties of Mg-(RE)-Zn alloys under different conditions were investigated, i.e. as-cast alloys and annealed alloys at 560 oC for 7 h. The results show that the as-cast Mg-(RE)-Zn alloy contains mainly the α-Mg(RE) solid solution and the GdMg phase, which distributes predominantly in the eutectic structure, but the eutectic microstructure completely disappears and there is no deposition in the grain boundary after annealing at 560 oC for 7 h. The annealed Mg-(RE)-Zn alloy involving the α-Mg (RE) phase, the MgGd3 phase and the MgZn2 phase, has a higher hardness, a higher ultimate tensile strength and a better elongation compared with the as-cast Mg-(RE)-Zn alloy. There is a significant difference in the fracture mode for the two alloys after a uniaxial tensile test. The fracture of the as-cast Mg-(RE)-Zn alloy exhibits an intergranular mode, while the fracture of the annealed Mg-(RE)-Zn alloy shows a transgranular mode induced by microvoid coalescence