Recently, there has been increasing interest in the transformation process between 18R and 14H long period stacking ordered (LPSO) structures in Mg-Y-Zn alloys. However, the detailed phase transformation associated with 18R and 14H structures remains to be established. In this work, the effects of high temperature annealing on the microstructure evolutions of LPSO structure of ascast and as-extruded Mg 97 Y 2 Zn 1 alloy were investigated by OM, SEM and TEM. The results show that the as-cast

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... y is mainly composed of network-shaped 18R-LPSO phase, stacking faults (SFs), α-Mg matrix and a small number of Mg 24 Y 5 particles. After extrusion, the 18R phase is rearranged in lines along the direction of extrusion and plenty of fine 14H lamella is precipitated in the matrix. During the early stage of annealing, the 14H phase in the as-cast alloy is nucleated massively in the SFs regions around 18R structure. With transformations from the 18R phase, the 14H lamella develops along the directions of length and thickness, and their volume fraction reaches to the maximum when the alloy is annealed for 30 h. As the annealing process continues, the dissolution of 14H phase into the