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.