Electro-reduction of spent nuclear oxide fuels in molten salt was the key step of pyroprocessing for oxide fuel treatment. In the present study, theE-pO2– diagram for rare earth elements in molten LiCl-KCl at 450 oC were developed based available experi-mental data.E-pO2– 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 availableE-pO2– 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.