A thermodynamic modeling and optimization of the La–Mg system is carried out by means of the CALPHAD method taking into account the latest experimental results. The liquid, bcc-La, fcc-La, dhcp-La and hcp-Mg solutions are modeled as substitutional solutions (La, Mg) using the Redlich–Kister formalism. The LaMg, LaMg 2 , La 5 Mg 41 , La 2 Mg 17 and LaMg 12 phases are treated as stoichiometric compounds, and the non-stoichiometry of LaMg 3 is described as (La,Mg) 0.25 Mg 0.75 . The results are in good agreement with the set of experimental data which were carefully discussed and selected.