Microstructure and mechanical properties of Mg-Zn-Er-Zr alloys were characterized in detail. The grain size of as-cast Mg-Zn-Er-Zr alloy was greatly decreased by the Mg-Zn-Er phases formed at grain boundaries. The addition of Er can increase the yield strength (YS) but decrease the ultimate tensile strength (UTS) and elongation of as-cast Mg-Zn-Zr alloy. The thermally stable Mg-Zn-Er phases were just partially dissolved into the matrix during solution treatment. And the addition of Er can prolong the precipitation process of Mg-Zn-Zr alloy. Solution-plus-ageing treatment can increase the strength of both the Mg-Zn-Zr and Mg-Zn-Er-Zr alloys, but the strengthening effect of Mg-Zn-Er-Zr alloy was greatly weakened, for the incompletely solution of Mg-Zn-Er phases. Er can greatly enhance the high temperature elongation of Mg-Zn-Zr alloy, but the increase of high temperature tensile strength was just a little.