► Effect of heat treatment on microstructure and mechanical property of GWZK alloy is investigated. ► Volume fraction and distribution of RE precipitates is modulated by controlling formation of 14H-LPSO structure. ► The best mechanical property is obtained from the alloy treated at 793K×10h+498K×16h. ► The performance is affected by optimization for combination between 14H-LPSO structure and RE precipitates. The effects of solid solution and aging treatments on the microstructure evolution and mechanical property of the as-cast Mg–14Gd–3Y–1.8Zn–0.5Zr alloy are investigated in this study. The microstructures of the 14H type long period stacking ordered (LPSO) structure and RE precipitates in the conditioned alloys are observed and analyzed by XRD, SEM and TEM. The mechanical properties of the alloys are evaluated Vickers hardness and ambient-temperature tensile tests. The results show that the volume fraction of 14H-LPSO structure in the alloy increased gradually with continued solution. The maximum volume fraction of RE precipitates is obtained when the alloy is solid-solution-treated at 793K for 10h and subsequently aged at 498K for 16h, under this optimum condition the mechanical properties (ultimate tensile strength: 366MPa, yield strength: 230MPa, elongation: 2.8%) at ambient temperature are achieved. The good performance of the studied GWZK alloy results from the dense precipitation of 14H-LPSO structure and RE precipitates in the matrix during the solid solution and aging process.