The ultrafine TiO2 nanoparticles (NPs) have been successfully prepared through a facile one-pot hydrothermal approach by stearic acid modification. The as-prepared ultrafine TiO2 NPs with average particle size of 5.9 nm have been fabricated into oil-based nanofluids, which exhibit significantly enhanced thermal conductivity and long term stability. The maximum thermal conductivity enhancement of about 44.6% was achieved at 25 degrees C with the 0.05% volume fraction of TiO2 NPs. The effect of nanoparticle loading and temperature on the thermal conductivity of the nanofluids was also carefully investigated. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.