The microstructure, phase composition, and mechanical properties of Mg-9Er-6Y-xZn-0.6Zr (x = 1, 3, 5 wt%; nominal chemical composition) series alloys were investigated through optical microscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry, transmission electron microscopy, and tensile tests. Numerous granular Mg-24(Er, Y, Zn)(5) phases were distributed in a discontinuous network mainly along the grain boundaries in the alloy with 1 wt% Zn. With increasing Zn content, the Mg-24(Er, Y, Zn)(5) phases in the alloys gradually disappeared, the amount of block Mg12Zn(Y, Er) phases increased, and the block size became larger. In addition, a few lamellar phases grew parallel with one another from the grain boundaries to the grain interior in the alloys. The crystallographic structures of the Mg12Zn(Y, Er) and Mg-24(Er, Y, Zn)(5) phases were confirmed as 18R-type long-period stacking ordered structures and body-centered cubic structures, respectively. The Mg12Zn(Y, Er) phases with long-period stacking ordered structures increased the strength and toughness of the alloys more than the Mg-24(Er, Y, Zn)(5) phases with body-centered cubic structures.