Molten salt is an important working fluid in the field of solar energy. However, the mixed convection heat transfer of molten salts has not been investigated. In this study, the performance of the mixed convection heat transfer of molten salt in horizontal square tubes under different side heating conditions is experimentally and numerically examined. The bottom side heating is experimentally studied, and a correlation is fitted on the basis of the experimental data. A good agreement between the correlations and the experimental data is achieved. The experimental results show that the mass flow rate of molten salt and the Reynolds number (Re) increase with the increase in the pump frequency. Moreover, the Nusselt number (Nu) increases with the increase in Re. The maximum enhancement of Nu in the mixed convection heat transfer compared with the forced convection heat transfer is approximately 50%. The experimental results and previous simulation results of the mixed convection heat transfer of molten salt with bottom surface heating are also compared. The comparison results also show a good agreement. The two-side heating condition (bottom and lateral side) is numerically examined. The simulation results of the two-side heating show that the vortexes caused by the buoyancy effect change the shape of the mainstream core area and strengthen the performance of heat transfer. (C) 2017 Published by Elsevier Ltd.