Electro-reduction of spent nuclear oxide fuels in molten salt was the key step of pyroprocessing for oxide fuel treatment. In the present study, the E-pO~(2–) diagram for rare earth elements in molten Li Cl-KCl at 450 oC were developed based available experimental data. E-pO~(2–) diagrams could show the stability of each chemical compound in the salt, and therefore, the diagrams could be applied to predict experimental conditions for electro-reduction of spent nuclear fuel efficiently. Compared with the available E-pO~(2–)diagrams, the present study concerned the activity coefficient of the element studied in the molten salt in all reactions, which made the diagram be more reliable and accurate. Electro-reduction of spent nuclear oxide fuels in molten salt was the key step of pyroprocessing for oxide fuel treatment. In the present study, the E-pO ~ (2-) diagram for rare earth elements in molten LiCl-KCl at 450 ° C were developed based on available data. E-pO ~ (2-) diagrams could show the stability of each chemical compound in the salt, and therefore, the diagrams, the diagrams could be applied to predict experimental conditions for electro-reduction of spent nuclear fuel efficiently. Compared with the available E-pO ~ (2-) diagrams, the present study concerned the activity coefficient of the element studied in the molten salt in all reactions, which made the diagram be more reliable and accurate.