Thermal conductivity and stability of carbon nanotube (CNT) structures in water-based nanofluid, as well as their dependence to temperature and time variation are of a great concern. In order to investigate such dependence, five different structures, namely SWNT (single wall CNT), DWNT (double wall CNT), FWNT (few wall CNT) and two different multiwalls were applied in this study. The experiments reveal that the maximum UV-VIS absorbance of the solution corresponds to the dispersion of SWNT in the base fluid. The results from zeta size distribution and thermal conductivity demonstrate that as the number of nanotube wall increase, both stability and thermal conductivity decrease. (C) 2011 Elsevier Ltd. All rights reserved.