Abstract: In the temperature range from 923 to 1223K, the two-phase equilibrium relationships between solid CaO, molten CaCl2 and CaCl2-NaCl were estabilished seperately. The solubilities of CaO in CaCl2 and equimolar CaCl2-NaCl were measured by chemical method and, according to the rules of phase equilibrium, the activity coefficient, excess Gibbs free energy, enthalpy and entropy of CaO in the saturated CaO molten salts were calculated. In the temperature range from 1123 to 1223K, the solubility of CaO in CaCl2 is between 14% and 16%, i.e., higher than that in CaCl2-NaCl. The activity coefficient of CaO in molten CaCl2 is smaller than that in CaCl2-NaCl, i.e., CaCl2 attracts CaO more than what CaCl2-NaCl does. The behavior of CaCl2-CaO where CaO is saturated is close to regular solution. The relationships between the solubility and activity coefficient of CaO in molten salts and temperature are obtained through linear fitting.