Precipitates formed in an Mg–0.5at%Ce alloy during age-hardening at certain temperatures ranging between 180°C and 250°C have been thoroughly investigated by a combined technique of TEM and HAADF-STEM. The precipitation sequence can be presented as Mg-solid solution→GP-zone→β 1 (Mg 3 Ce; BiF 3 -type)→β (Mg 12 Ce; Mn 12 Th-type). At an early stage of aging (180°C for 2h), fine precipitates of planar GP-zone with an ordered structure appear in parallel to (100) m planes of the Mg-matrix, having a thickness of sub-nm and an extension of 5–15nm. With an advance of aging, the GP-zones increasingly grow larger and combine with the neighbors. When the GP-zones have reached as large as approximately 20nm in diameter, the age-hardening effect is maximized (180°C for 70h). After the top-aging, the GP-zones begin to decompose and disappear, and instead precipitates of two other phases, i.e. the β 1 - and the β-phase come into sight in the matrix with definite crystallographic orientation relations of [001] m //[110] β 1 //[100] β and (110) m //(111) β 1 //(010) β , finally completing the aging effect with full of coarse β-precipitates.