Dispersing　nanomaterials　can　effectively　improve　the　thermal　energy　storage　performance　of　molten　salts.　However,　research　on　such　improvement　mechanism　is　still　immature,　as　the　understanding　of　the　performance　improvement　levels　induced　by　different　nanoparticles　has　not　been　consistent.　Using　silica　nanoparticles　as　additives,　this　work　prepares　nine　molten　salt　nanofluids　by　aqueous　solution　method　with　the　use　of　potassium　nitrate,　sodium　nitrate,　and　their　binary　mixture　as　base　salts.　The　key　thermal　performance　of　these　nanofluids　are　evaluated　through　different　characterization　approaches,　such　as　differential　scanning　calorimetry,　thermal　gravimetric　analysis,　laser　flash　analysis,　and　scanning　electron　microscopy　for　microstructural　characteristics.　The　possible　performance　enhancement　mechanism　caused　by　silica　nanoparticle　is　also　explored.　Results　show　that,　for　these　nine　molten　salt　nanofluids　containing　silica　nanoparticles,　the　change　in　the　melting　point　is　negligible.　The　potassium　nitrate　salt　dispersed　with　20-nm　silica　nanoparticles　achieve　the　highest　performance　improvement　not　only　in　latent　heat　but　in　specific　heat　and　thermal　conductivity,　while　sodium　nitrate　salt　nanofluids　reach　maximum　improvement　at　their　decomposing　temperature.　The　improvement　level　can　be　associated　with　the　electronegativity　of　alkali　ions.　Cloud　nuclei　cause　microstructure　differences　among　molten　salt　nanofluids　and　may　be　the　main　reason　for　the　thermal　performance　enhancement　of　the　molten　salt　nanofluids.