Abstract: This work concerns the electrochemical behaviors of La³⁺ on W electrodes in the LiCl-KCl-LaCl₃ molten salt at 723-813 K. The results show that La³⁺ is reduced at about -2.04 V(vs. Ag/AgCl) on inert W electrode, which is a one-step three-electron transfer process. The redox potential, Gibbs free energy and activity coefficient of La³⁺/La are calculated by open circuit chronopotentiometry on W electrode in LiCl-KCl-LaCl₃ molten salt. Potentiometric titration was used to study the reaction of lanthanum chloride with oxide ions. The titration curve shows that the precipitate of oxide is LaOCl. According to the experimental apparent electrode potential, activity coefficient and relevant thermodynamic data, the phase diagram of La-O stability was drawn. The stable phase diagram of E-pO^2- shows that LaOCl is the most stable compound in the range of  higher O^2- concentration at 723 K.