Abstarct: Nanofluid is prepared by stable dispersing of nanoparticles in baxse fluid. In this thesis, the convective heat transfer and pressure drop characteristics of alumina/water and titania/water nanofluids flowing inside a horizontal circular tube were studied experimentally. To do this, a flow loop facility was constructed to conduct the experiments that the assumed boundary condition of the test section was the constant heat flux in the wall. The Al2O3 nanoparticles in 40 nm and the TiO2 nanoparticles in 15 nm were prepared and dispersed in distilled water to form stable suspension containing 0.1%, 0.5%, 1.0%, 1.5% and 2.0% volume concentrations of nanofluids. Results indicated that heat transfer and Nusselt number increase with addition of nanoparticles and the enhancement increases with increasing the volume concentration. The maximum enhancement of the Nusselt number was about 22% at Re=13500 using 2% alumina nanoparticles. However the enhancement for titania nanoparticles was approximately 8% at Re=11800. One differential pressure transmitter is also employed to measure the differential pressure between inlet and outlet of the tube. The measured pressure drop using nanofluids is almost equal to that of the baxse fluid. These experiments are also performed with water and the results compared to those obtained with nanofluids. Moreover, the experimental results are also compared to predictions made using the existing single-phase correlations of convective heat transfer coefficient and pressure drop in turbulent regime. In general, if the measured temperature and valume dependent thermal conductivities and viscosities of the nanofluids are used in calculating the Reynolds, Prandtl, and Nusselt numbers, the existing correlations accurately predict the convective heat transfer and pressure drop behavior in tubes.