High strength magnesium alloys with a long period stacking ordered (LPSO) structure have been developed in Mg-Zn-Y alloys. The Mg-Zn-Y alloys are mainly consisted of two phases, which are LPSO and ot-Mg phases, and their mechanical properties are improved drastically by plastic deformation. In this study, we have tried to construct a constitutive formulation of the yield strength for the extruded LPSO Mg_<97>Zn_1Y_2 alloy as a model alloy. The effect of extrusion ratio and speed on the yield strength and microstructure of the LPSO Mg_<97>Zn_1Y_2 alloy has been investigated. The extruded LPSO Mg_<97>Zn_1Y_2 alloy was composed of three regions which were LPSO phase, dynamic recrystallized and un-recrestallized regions of α-Mg phase. On the basis of a composite model consisted of three regions, a constitutive formulation of the yield strength of the extruded LPSO Mg_<97>Zn_1Y_2 alloy has been constructed from the investigations of the microstructure and mechanical properties of extruded alloys with a singe LPSO phase and a single α-Mg phase. The yield strength calculated from the constitutive formulation represented the experimental yield strength with an accuracy of 7%. This result reveals that the composite model can be applied to the yield strength of LPSO magnesium alloys.