Recently, the Mg-Y-Zn alloy systems have received great attention due to their unique microstructures and excellent mechanical properties. Three kinds of ternary equilibrium Mg-Y-Zn phases have been reported in the systems: the W phase (Mg₃Y₂Zn₃), the I phase (Mg₃YZn₆) and the X phase (Mg₁₂YZn, long period stacking ordered (LPSO)structure). To further study the evolutions of LPSO structures in Mg-Y-Zn alloys, three Mg-(2, 3, 4)Y-1Zn (atomic fraction, %) ternary alloys were prepared by casting and extrusion. Based on the OM, SEM and TEM observations, the microstructures of the as-cast WZ21 and WZ31 alloys are mainly composed ofα-Mg and Mg₁₂YZn duplex microstructures, while that of the as-cast WZ41 alloy consists of α-Mg,Mg₁₂YZn and Mg₂₄Y₅ phases. The Mg₁₂YZn phase which forms a network is a kind of 18R-LPSO structures and the Mg₂₄Y₅ phase is inclined to be located within 18R phases. During homogenization treatment, part of 18R phase dissolves and 14H lamellas are precipitated in the matrix of the WZ21 and WZ31 alloys. After extrusion, the 18R phases are aligned along the extrusion direction, whereas the 14H lamellas in the matrix are still parallel to each other. During solution treatment (T4), the 18R structures continue to dissolve and 14H lamellas further develop. With increase of the Y/Zn atomic ratio, the volume fraction of 14H-LPSO phase increases after T4 treatment. The mechanical properties for the extruded alloys are better than alloys in as-cast, as-annealed and T4-treated stages. With increasing Y content, the strength of the alloys increases, but the ductility decreases. Tensile strength of the extruded WZ41 alloy reaches 350 MPa at room temperature.