This paper reports on the results of an experimental study on the heat transfer and flow performance of multiwalled carbon nanotubes (MWCNT) in aqueous suspensions flowing in multiport minichannel flat tubes with smooth/micro-fin surface under a constant heat flux condition. The MWCNT-water nanofluids volume concentrations are 0.001%, 0.005%, 0.01%, and 0.1%. Results indicate that the friction factor and the Nusselt number of the nanofluids are higher than those of water. The Nusselt number does not increase with volume concentration. The heat transfer of nanofluids in the micro-fin tubes is higher than that of nanofluids in the smooth tube. However, the heat transfer enhancement of nanofluids in the micro-fin tubes is lower than that of nanofluids in the smooth tube. The effect of micro-fin spacing is more evident in nanofluids than in water. Nanofluids with a concentration of 0.01% flowing in the tube with closely spaced micro-fin tube exhibit the optimal thermal performance in accordance with the performance evaluation criterion. (C) 2016 Elsevier B.V. All rights reserved.