A novel as-cast Mg96.82Gd2Zn1Zr0.18 alloy was produced by conventional ingot metallurgy. By scanning electron microscope and transmission electron microscope observations, its as-cast microstructure is mainly composed of the alpha'-Mg solid solution, the coherent fine-lamellae and the eutectic. The beta-phase ((Mg,Zn)(3)Gd) as the second phase in eutectic has a face-center cubic structure. While, the coherent lamellae consist of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure. At present, the lamellar 14H-type LPSO structure has first been observed within alpha'-Mg matrix in the as-cast Mg-Gd-Zn-Zr alloys. It can be concluded that the lamellar 14H-type LPSO structure within alpha'-Mg matrix is a special structure obviously different from alpha-Mg matrix (2H-type structure) in structure, composition, and formation condition.