The crystal structures and microstructures of precipitates formed in an Mg-0.5 at%Nd alloy aged at certain temperatures ranging between 170°C and 250°C are studied in detail by high-angle annular detector dark-field scanning transmission electron microscopy. The precipitation sequence can be presented as Mg-solution → GP-zone → β′ (orthorhombic) → β₁ (fcc). At the early stage of aging (170°C for 2 h), fine precipitates of planar GP-zones appear in parallel to (100)_m planes, with a thickness of sub-nm and a length of 5–15 nm (the subscript letter of m denotes matrix). With an advance of aging, the GP-zones increasingly grow larger and combine with the neighbours, thus making themselves further prolonged along the directions addressed above. When reaching at the top-stage of aging (170°C for 100 h), the alloy additionally allows the β′-phase to coexist, taking the form of lens-shape with a thickness of 2–5 nm and a diameter of 5–15 nm. The β′-phase has an orthorhombic structure (Mg₇Nd) with a=0.64 nm, b=1.1 nm, and c=0.52 nm, which is coherently connected to the matrix. At the stage of over-aging, both the GP-zones and the β′-phase disappear and instead coarse precipitates of the stable β₁-phase (Mg₃Nd; fcc) are formed with particular crystallographic relations of [001]_m||[110]_p and [110]_m||[112]_p (the subscript letter of p denotes precipitate).