Phase compositions, lattice structures and phase equilibria of Mg–Zn–La system in Mg-rich corner at 345°C have been investigated by equilibrated alloy method and diffusion couple technique. The results show that a binary solid solution of Mg 12 La is confirmed as (Mg,Zn) 12 La with 0–4.96at.% Zn. It has a body-centred tetragonal lattice structure with giant unit cell of a=b≈1.032nm and c≈7.748nm. Another linear ternary intermetallic (Mg,Zn) 11 La (Zn≈8.66–43.39at.%) has been identified too. (Mg,Zn) 11 La has a C-centred orthorhombic structure, and the unit cell of which becomes smaller with the increase of Zn content. The smallest lattice parameters of it are a≈1.1191nm, b≈0.9701nm and c≈0.9498nm. Both (Mg,Zn) 12 La and (Mg,Zn) 11 La are in equilibrium with Mg solid solution in their composition range. A three-phase equilibrium of Mg+(Mg,Zn) 12 La+(Mg,Zn) 11 La is also ascertained. In addition, MgZn phase containing about 1.53at.% La is confirmed, which is in equilibrium with Mg solid solution and (Mg,Zn) 11 La at 345°C. This result indicates that it is impossible for Mg 17 Zn 21 La 2 reported in literature to be in equilibrium with Mg solid solution as a stable phase at this temperature.