It has been proposed that a long-period stacking ordered (LPSO) structure is responsible for the excellent mechanical properties of lightweight alloys of Mg-Y-RE (RE: rare earth elements) system. In this study, the phenomenological simulation model for describing the formation process of the LPSO structure in Mg–Y–Zn system is constructed by means of the phase-field method. The results obtained are as follows: The proposed phenomenological model is useful for describing the formation process of LPSO structure, where the enrichment of solute elements Zn and Y on stacking fault region induces the growth of the stacking fault itself. This model is able to calculate not only the formation of lamella structure of LPSO phase but also the growth of single stacking fault region in LPSO phase, simultaneously. It is suggested that the lamella spacing of LPSO structure may depend on the initial condition when a LPSO phase nucleates.