Phase equilibria involving the long-period stacking ordered phases including 14H,18R and 10H in the Mg-rich corner of the Mg-Y-Zn system at 400 and 500℃ have been experimentally investigated by using X-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).The coexistence of 14H and 18R as well as 18R and 10H was confirmed from atomic scales.The phases 14H,18R and 10H were all stable phases from 400 to 500℃.The experimentally proved three-phase equilibrium of 14H,18R and α-Mg instead of 14H,18R and Mg24Y5 were presented in the modified isothermal sections.The latter three-phase equilibrium was reported in the available literature.The modified isothermal sections are conducive to guide the composition design to obtain the alloys with favorable microstructure constituents and mechanical properties.