We synthesis CuO nanoparticles of average diameter 8 nm by a simple precipitation technique and study the thermal properties of the suspensions. The thermal conductivity enhancement observed in water and ethylene glycol based nanofluids with 1 vol.% CuO nanoparticles loading are 31.6 and 54%, respectively. The large enhancement in thermal conductivity is attributed to the finer particle size and monodispersity of nanoparticles. It has been found that the thermal conductivity of the nanofluid increases nonlinearly with the volume fraction of nanoparticles. The time-dependent thermal conductivity in water based CuO nanofluid shows that the thermal conductivity decreases with elapsed time due to clustering of nanoparticles with time, as confirmed microscopically. The experimental results show that the nanoparticles size, polydispersity, cluster size and the volume fraction of particles have a significant influence on thermal conductivity. (C) 2007 Elsevier B.V. All rights reserved.