In this research natural convection heat transfer of non-Newtonian nanofluids in two cylindrical geometries were investigated experimentally. Two kind of non-Newtonian nanofluids were prepared by dispersion of Al 2 O 3 and TiO 2 nanoparticles in a 0.5% wt. aqueous solution of carboxymethyl cellulose (CMC). Results show that the heat transfer performance of nanofluids is significantly enhanced at low particle concentrations. Increasing nanoparticle concentration has a contrary effect on the heat transfer of nanofluids, Indeed for both nanofluids there exists an optimum nanoparticle concentration that heat transfer coefficient passes through a maximum. It is also observed that the heat transfer enhancement of TiO 2 nanofluids is higher than that of the Al 2 O 3 nanofluids. The effect of enclosure aspect ratio was also investigated. As expected, the heat transfer coefficient of nanofluids as well as the base fluid increases by increasing the aspect ratio. Keywords : Nanofluids, Natural convection, Non-Newtonian Nanofluids