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 Mg12La is confirmed as (Mg,Zn)12La with 0-4.96 at.% Zn. It has a body-centred tetragonal lattice structure with giant unit cell of a = b ≈ 1.032 nm and c ≈ 7.748 nm. Another linear ternary intermetallic (Mg,Zn)11La (Zn ≈ 8.66-43.39 at.%) has been identified too. (Mg,Zn)11La 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 ≈ 1.1191 nm, b ≈ 0.9701 nm and c ≈ 0.9498 nm. Both (Mg,Zn)12La and (Mg,Zn)11La are in equilibrium with Mg solid solution in their composition range. A three-phase equilibrium of Mg+(Mg,Zn)12La+(Mg,Zn)11La is also ascertained. In addition, MgZn phase containing about 1.53 at.% La is confirmed, which is in equilibrium with Mg solid solution and (Mg,Zn)11La at 345 °C. This result indicates that it is impossible for Mg 17Zn21La2 reported in literature to be in equilibrium with Mg solid solution as a stable phase at this temperature.