Abstract Nanofluids show clearly a very interesting alternative for working fluids of microchannels. This paper presents an experimental study on heat transfer and flow features of Al 2 O 3 –water nanofluids through a circular microchannel with the 0.5 mm inner diameter. Al 2 O 3 –water nanofluids with particle volume concentrations of 0.25%, 0.51% and 0.77% are prepared. The effect of particle concentration and Reynolds number on the heat transfer and flow features of nanofluids-cooled microchannel are examined and then compared with the data for water-cooled microchannel. The results show that Nusselt number of Al 2 O 3 -water nanofluids is higher than that of deionized water, and increases with increasing Reynolds number and particle concentration. The maximum heat transfer enhancement achieves 10.6% in the test conditions. Meanwhile, the enhancement effect is more apparent at higher Reynolds number in the range of laminar flow. A Nusselt number correlation for nanofluids flowing in microchannels is proposed, and the predicted Nusselt numbers agree with the experimental data with deviations of −4% and +5%. The friction factor of the nanofluids has an increase within 7.9% compared with that of deionized water. A friction factor correlation is proposed, and the predictions agree with the experimental values with deviations of −3% and +5%.