Molten salts and their mixtures are widely used as heat transfer fluid in the research area of solar energy applications. Measured viscosities of molten salts often show large discrepancies due to difficulties of measurement at elevated temperatures, especially for molten salt mixtures. In this paper, a theoretical model is presented for predicting the viscosity of binary molten salts, in which only data of viscosity, density and fraction of each component salt are needed. Comparisons with experimental data show that the model gives prediction of viscosity with errors less than 9.94%, though part of this error may probably come from experiment. In comparison, the Eyring equation always gives larger errors, reaching 11.4% in the worst case. The present work indicates that the model shows improved ability in prediction of viscosity of molten salt mixtures. (C) 2016 Elsevier Ltd. All rights reserved.