By means of optical microscope(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM)and transmission electron microscopy(TEM) analyses, the microstructures of as-cast and heat-treated Mg–4Zn–1Y(wt%) alloy containing quasi-crystal phase were studied.The microstructure of the as-cast alloy consists of a-Mg solid solution grains, intermetallic particles and eutectic phases(W-phase and I-phase), and huge grains with serious dendritic segregation are clearly observed. After heat treatment, phase transformation and dissolution occur in the alloy and many phases remain. When the alloy was treated above 410 °C, the eutectic phases transform into spherical shape as the I-phase turns to W-phase. After heat treatment for long time, the alloy is over burnt and the W-phase decomposes to Mg–Y binary phase. By means of optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyzes, the microstructures of as- cast and heat- treated Mg- 4Zn- 1Y (wt% ) alloy containing quasi-crystal phase were studied. The microstructure of the as-cast alloy consists of a-Mg solid solution grains, intermetallic particles and eutectic phases (W-phase and I-phase), and huge grains with serious dendritic segregation are clearly observed. After heat treatment, phase transformation and dissolution occur in the alloy and many phases remain. When the alloy was treated below 410 ° C, the eutectic phases transform into spherical shape as the I-phase turns to W-phase. treatment for long time, the alloy is over burnt and the W-phase decomposes to Mg-Y binary phase.