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 as-cast and as-extruded Mg₉₇Y₂Zn₁ alloy were investigated by OM, SEM and TEM. The results show that the as-cast alloy is mainly composed of network-shaped 18R-LPSO phase, stacking faults (SFs), α-Mg matrix and a small number of Mg₂₄Y₅ 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 matrix proceeds at the same time. There is almost no 14H lamella present in the center ofα-Mg matrix but a few remains near the 18R phase in the microstructure of the specimen annealed for 200 h. In case of the as-extruded alloy, however, abundant fine 14H lamella has already been introduced during the hot extrusion. When the annealing treatment is operated, the 18R phase keeps dissolving into the matrix until disappearing completely, while the 14H lamella grows continuously. When the whole matrix is covered by 14H lamella, the development of 14H lamella along the length direction is hindered by the grain boundaries. And only coarsening 14H lamella is observed in the as-extruded alloy sample annealed for 200 h.