Nanofluid is a kind of new engineering material consisting of nanometer-sized particles dispersed in base fluid. In this study, various nanoparticles, such as multi-walled carbon nanotube (MWCNT), fullerene, copper oxide, and silicon dioxide have been used to produce nanofluids for enhancing thermal conductivity and lubricity. As base fluids, DI water, ethylene glycol, and oil have been used. To investigate the thermo-physical properties of nanofluids, thermal conductivity has been measured. The experimental results of thermal conductivity of nanofluids are compared with the modeling results predicted by Jang and Choi model [14]. The stability of nanofluid has been estimated with UV-vis spectrophotometer. Thermal conductivity of nanofluid has been increased with increasing volume fraction of nanoparticle except for water-based fullerene nanofluid which has lower thermal conductivity than that of base fluid due to its lower thermal conductivity, 0.4 W/mK. Stability of nanofluid has been influenced by the characteristics between base fluid and suspended nanoparticles. (C) 2007 Elsevier B.V. All rights reserved.