Precipitations in Mg–Li–Zn ternary alloys containing 4 to 13%Li and 4 to 5%Zn (in mass%) with α or β single phase, or with (α+β) dual phases were investigated using a micro-Vickers hardness measurement and transmission electron microscopy. Age hardening in the α phase alloy was found to occur, which was attributed to the precipitation of the stable θ (MgLiZn) phase with the following orientation relationships: [10\\bar10]_α||[110]_θ, (0001)_α||(1\\bar1\\bar1)_θ. In the (α+β) phases alloy, the precipitation of the α phase together with the metastable θ′ (MgLi₂Zn) phase occurred at grain boundaries between the α and β, and also β and β grains. The orientation relationships between the α and θ′ were as follows; (0001)_α||(01\\bar1)_θ′, [0\\bar110]_α||[111]_θ′. Age hardening in the β alloy was caused by the precipitation of the θ′ phase and over-aging was attributed to the precipitation of the α and θ phases.