At present, long period stacking ordered (LPSO) structures in Mg-RE-Zn (RE=Y, Dy, Ho, Er, Gd, Tb, Tm) alloys have been focused on. According to some reports, Mg-Gd-Zn alloys are classified as type II, i.e., there are no LPSO structures in as-cast alloys, but LPSO structures appear after heat treatment. To further study the formation of LPSO structure in Mg-Gd-Zn(Zr)\linebreak alloys, a Mg_96.32Gd_2.50Zn_1.00Zr_0.18 alloy was prepared by ingot metallurgy (I/M) in this work. Based on OM, SEM and TEM observations, the as-cast microstructure of Mg-Gd-Zn-Zr alloy consists of α-Mg solid solution, lamellar 14H-type LPSO structure within α-Mg grains and dendritic eutectic structure at grain boundaries, in which the eutectic phase is the β-phase ((Mg, Zn)₃Gd)), thus, Mg-Gd-Zn(Zr) alloys should be attributed to type I, i.e., there are LPSO structures in as-cast alloys. During solid solution treatment at 500 ℃ for 35 h, a solid transformation, dendritic β-phase with fcc structure→lamellar X-phase with 14H structure, was observed at grain boundaries. During subsequent peak-aging treatment at 200 ℃ for 128 h, ellipsoidal β´ and rhombus β₁ phases precipitated within α-Mg grains. Tensile tests at room temperature and Vickers hardness tests show that the alloy solution-treated and aging-treated has higher mechanical properties, i.e., σ_b=290.7 MPa, σ_s=162.5 MPa, δ=10.35% and 108.0 HV. The improvement of mechanical properties are attributed to the composite strengthening-and-toughening effect of the 14H-type LPSO structures, and the β´ and β₁ aging phases.