Molten salt is popular as heat transfer fluid in solar energy applications in the temperature level of hundreds of degrees or higher, but it would be a tedious work to measure the thermal conductivity at elevated temperature due to convective and radiative effects. In this paper, a theoretical model is presented for predicting the thermal conductivity of binary molten salts, in which only data of thermal conductivity, density and mass fraction of each component salt are needed. Comparisons with experimental data show that the model gives prediction of thermal conductivity with errors less than 6.4%. The linear mixing rule, however, always gives large positive errors, especially at about 0.5-0.5 mol fraction, which may sometimes reach 33.8%. The present work indicates that accurate prediction of thermal conductivity of molten salt mixture, even at elevated temperature, can be achieved if the thermal conductivity of pure compounds potentially applied in molten salt mixtures is measured with enough accuracy. (C) 2015 Elsevier Ltd. All rights reserved.