Heating and cooling are the most important technical challenges facing many industries including power station, production processes, traortation and electronics. The conventional methods for enhancing heat transfer rate such as heat surface addition (fins), vibration of heated surface, injection or suction of fluid and applying electric or magnetic field can hardly meet the requirement of increase in heat flux. There is therefore an urgent need for new and innovative coolants with improved heat performance. The fluids with nanometer-sized solid particles suspended in them are called nanofluids. These fluids can be employed to increase the heat transfer rate in various applications. In this study the convective heat transfer for , and nanofluids through a circular tube was investigated experimentally. The flow was laminar and constant wall temperature was used as thermal boundary condition. The Nusselt number of nanofluids for different nanoparticles concentrations as well as various Peclet numbers were obtained. Also the rheological properties of the nanofluid for different volume fractions of nanoparticles were measured and compared with theoretical models. . Results emphasize the enhancement of heat transfer, which is due to the nanoparticles present in the fluid. The experimental results also show that heat transfer coefficient is enhanced by increasing the nanoparticles concentration as well as Peclet number.