Rheological and thermal behaviors of nanofluids with non-Newtonian base fluid were investigated experimentally. A solution of 0.5% wt. carboxymethyl cellulose in de-ionized water was used as the base fluid. ?-Al 2 O 3 , CuO and TiO 2 nanoparticles were employed to prepare three kinds of nanofluids. Rheological characteristics of the base fluid and ?-Al 2 O 3 , CuO, and TiO 2 nanofluids with different concentrations and temperature range of 5-50 °C were measured using a cylindrical rheometer. The base fluid and all nanofluids exhibited pseudoplastic behavior. Flow behavior index of nanofluids is less than that of the base fluid and decreases with nanoparticle concentration. At a given particle concentration flow behavior index of nanofluids increases with increasing temperature. Consistency index of nanofluids is larger than that of the base fluid and decreases with temperature. Thermal conductivities of the base fluid and those of the nanofluids with various concentrations and temperatures were measured. Thermal conductivity of nanofluids shows enhancement in comparison to that of the base fluid. Convective heat transfer coefficients of the base fluid and nanofluids in a circular straight horizontal tube in concentration range 0.1-1.5% vol. with two different boundary conditions of constant wall temperature and constant wall heat flux were obtained. Both laminar and turbulent flow regimes were considered. Results of all cases show that the average heat transfer coefficients of nanofluids are significantly larger than the base fluid. The heat transfer coefficient of nanofluids increases with increase in both Peclet number and particle concentration. Enhancement of heat transfer coefficient of nanofluids increases with particle concentration. In the case of constant wall temperature, enhancement of heat transfer coefficient of nanofluids in turbulent flow is greater than that of the laminar flow, but for constant wall heat flux condition there exists no significant difference. Peclet number has no significant effect on the relative heat transfer coefficient of nanofluids. This means that by increase in Peclet number heat transfer coefficients of the base fluid and that of the nanofluid increase with the same order. For constant wall heat flux boundary condition, the enhancement of local heat transfer coefficient of the base fluid and that of the nanofluid increases with increase in axial distance from the tube inlet. Results of local heat transfer coefficients of nanofluids show that the entrance length of nanofluids is larger than the base fluid and increases with increase in particle concentration. To predict the average heat transfer coefficients of nanofluids with different boundary conditions, two equations were obtained by an adequate non-linear curve fitting procedure.