This study experimentally investigated TiO2-water nanofluid single-phase flow and heat transfer characteristics in a multiport minichannel flat tube with a 1.65 mm hydraulic diameter. TiO2 nanoparticles with nominal diameters of 10, 30, and 50 nm were dispersed in water at various volume concentrations ranging from 0.005% to 1%. The friction factor and Nusselt number of the nanofluids were tested and compared with those of the base fluid, the Reynolds number of which varied from 100 to 6100. Results indicated that friction factor and Nusselt number of the nanofluids were higher than those of water. Both nanofluid density and particle migration significantly affected friction factor. Meanwhile, a slightly earlier laminar-turbulent transition was found for the nanofluids. Nu did not increase with increasing volume concentration and decreased with increasing particle diameter (4). The effects of thermal conductivity, viscosity, and chaotic movement of nanoparticles on heat transfer were discussed in this study. Nanofluids at 0.01% concentration with d(p) = 10 nm exhibited the best performance on the basis of the evaluation criterion, which considered heat transfer enhancement at the consequence of increased friction factor. (C) 2014 Elsevier Ltd. All rights reserved.