The β′ phase precipitated in a 95 at%Mg-5 at%Gd (Mg₉₅Gd₅) alloy aged in a peak hardness condition (at 200°C for 100 hrs) is studied by high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic-scaled HAADF-STEM observations of the β′ phase can propose a new structure model with an orthorhombic unit cell of a=0.64 nm, b=2.28 nm and c=0.52 nm and a composition of Mg₇Gd. The β′ precipitates have a plate-shape with an about 40 nm width along the [001]_m of the Mg-matrix and an about 100 nm length along [210]_m-typed directions, and joining of the plate-shaped precipitates establishes a two-dimensional cell structure parallel to the (001)_m plane of the Mg-matrix.