Therminol 55’ is one of the heat transfer fluids used in process industries and possesses low freezing andhigh boiling points. The thermal conductivity of Therminol 55 can be improved through dispersion ofnanoparticles leading to nanofluids. Magnesium oxide (MgO) nanoparticles (25e45 nm) were synthesizedfrom magnesium nitrate by reaction with ammonium carbonate at 80 C, followed by calcination at800 C for 1 h. MgO-Therminol 55 nanofluid was prepared by dispersing MgO nanoparticles in Therminol55 using oleic acid as dispersant, followed by bath ultrasonication. Thermal conductivity ratio ofnanofluids was found to vary linearly with nanoparticle volume fraction, while exhibiting biphasic trendwith respect to the influence of temperature. Liquid layering and micro-convection caused by Brownianmotion were found to be dominant contributors of thermal conductivity enhancement at lower (10e30 C) and higher (70e120 C) temperatures respectively. Our data reveal linear increase in overall heattransfer coefficient with nanoparticle volume fraction under constant bath temperature condition,closely matching the thermal conductivity e nanoparticle volume fraction relationship. This is the firstwork on preparation, characterization and heat transfer performance of MgO-Therminol 55 nanofluids,highlighting its value for energy management.