The purposes of the present study are to produce a high thermal efficient of oil-based nanofluids which can be used as ultra-high voltage transformer oil, and to investigate their thermal and physical properties under static and dynamic conditions. Three kinds of nanotluids are prepared by dispersing Al2O3 or AIN nanopartic1es in transformer oil. The thermal conductivities of the nanoparticles-oil mixtures increase with temperature, particle volume concentration and thermal conductivity of solid particle itself. It was quite important to eliminate H20 as byproducts of esterification and excess oleic acid which did not form stable chemical bonds with powder surface to get high dispersion stability.