Characterizations of phases in Mg-10%Y-5%Gd-2%Zn-0.5%Zr (WGZ1052) alloy during heat treatments were investigated by OM, XRD, SEM and EDS. The mechanisms of microstructure evolution were discussed. The results show that, after high temperature heat treatments, the Mg12ZnY phases still exist. During solution-treatment at 535 ℃, the amount of the long-period stacking order structures decreases. At 545 ℃ for 20 h and 24 h, there are still remnant Mg12ZnY compounds in the Mg matrix, the shape of which does not change and the amount does not decrease obviously.