This study investigated modulation of the long period stacking order (LPSO) structure in aged Mg 97 Zn 1 Y 2 alloys using conventional transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The irregular stacking sequence of a fragment of 24R-type LPSO acts as a catalyst for the transformation from 18R-to 14H-type LPSO. The elementary step of the transformation from 18R-to 24R-type takes place by the ledge-pair movement on different (0001) Mg planes with Shockley partial dislocations. Each ledge has a transition region in front of it. The transition regions are HCP-type stacking sequence with lower Zn and Y concentrations than those of the FCC-type enrichment layer. The solute elements migrate easily in the region, where solute elements produce a kind of diffusion field. Therefore, structural modulation occurs by a mechanism resembling diffusionaldisplacive transformation. Local strain analysis using HAADF-STEM images has elucidated that lattice spacing of (0001) Mg in the FCC-type enrichment layer is shorter than that in the HCP-type transition region. These structural and compositional irregularities are an elementary step in the transformation of LPSO in Mg 97 Zn 1 Y 2 alloys. A diffusionaldisplacive type transformation mechanism in LPSO has been proposed.