Carbon nanotubes have attracted much attention because of their unique structure and remarkable mechanical and electrical properties. One of the specific properties of carbon nanotube is unusually high thermal conductivity. In this study, we have prepared the nanofluids with high thermal conductivity by dispersion of multiwalled carbon nanotubes in water. Preparation stable and uniform dispersion of carbon nanotubes in the most fluids is so difficult because of the aggregation of carbon nanotubes. We were used the functionalization technique to produce the hydrophilic groups to the surfaces of carbon nanotubes and make a stable and uniform suspension of carbon nanotubes. During treatment MWNT with H 2 O 2 and mixture of HSO 4 and HNO 3 were attached hydroxyl and carboxyl groups to the surfaces of MWNT respectively. In order to disperse nanotubes in water, we were used ultrasonic generator . Hydroxylated MWNT in water have more thermal conductivity enhancement in comparison with the carboxylated one. Also we have studied the effect of concentration of carbon nanotubes and the duration of sonication to the thermal conductivity of nanofluids. The results show that the thermal conductivity enhancement increases with concentration. In addition, the duration of sonication strongly affects on the thermal conductivity of suspension and exits the optimum time sonication in which we have the maximum thermal conductivity enhancement. The results show that The optimum time of sonication decrease with concentration for 0.02 to 0.1 mass% of carbon nanotubes but at higher weight loading the optimum time is fixed at 10 min. SEM images show the forming of the three dimensional network of carbon nanotubes in the optimum time sonication cause to the more thermal conductivity enhancement and stability of nanofluids. We were obtained the variation of thermal conductivity of nanofluids after the sonication stopped to estimate the stability of suspensions. Although the thermal conductivity of suspension right after sonication increase by the increased concentration of carbon nanotubes but reduce the stability of suspension. We have also studied the thermal conductivity of mixture nanofluid containing titanium oxide and three kinds of carbon nanotubes. The TiO 2 nanoparticles may be agglomerated to the surface of carboxylated MWNT and increases the thermal conductivity of nanofluid but Hydroxylated MWNT entangle around TiO 2 nanocluster and form congeries so it causes a decrease in thermal conductivity enhancement. It is also possible that TiO 2 connected to the carbon nanotubes at the one or two end, when mixture.is heated.