Both face-centered cubic phases W′and W have close relationship with the icosahedral quasicrystal （ IQC） phase in Mg-Zn？Y alloys, but the crystal structure of W′ phase remains controversial. Based on electron diffraction analysis, energy dispersive X-ray spectroscopy and aberration-corrected scanning transmission electron microscopy （ STEM） , it can be concluded that the W′phase has a composition of Zn60？1 Mg23？9 Y16 and which is identified as the Cd45 Sm11？type structure. The W′ phase is composed of tetrahedral symmetric Friauf polyhedral clusters rather than icosahedral symmetric multiple-shell ones, so the W′ phase is not an approximant crystal of IQC. By in situ heating transmission electron microscopy （ TEM） , a phase transformation from W′to W was observed at 683 K in the as-cast alloy. The W′and W phases have a cube-to？cube orientation relationship, and the W′/W interface is coherent. Close correlation between their basic structural units, cuboctahedral and Friauf clusters, is believed to be responsible for the epitaxial nucleation and growth of W phase on the W′phase.