A metallothermic reduction technique was adopted to reduce rare earth oxides in a molten LiCl. A reducing agent of Li was supplied to the system by dissolving excess amount of Li metal on a separated section in a reactor. The extents of the reduction were measured by analyzing Li2O which is the product of the chemical reactions. Nine kinds of rare earth oxides in the form of RE2O3 were partially reduced to the corresponding metals until the Li2O concentrations reached certain critical values for each reaction. The Li2O activity coefficients were evaluated based on the measured critical concentrations. These experimental results gave important information concerning the chemical behavior of rare earth oxides during an electrochemical reduction process which will be utilized to reduce spent oxide fuel in a pyroprocessing scheme.