This study aims to evaluate the energy efficiency of nanofluid as a heat transfer fluid in a chaotic channel. To this end, hydrothermal characteristics of the water-Al2O3 nanofluid are numerically investigated in C-shaped and straight channels using single- and two-phase methods and then, the results are compared with each other. In the C-shaped channel, heat transfer and pressure drop show higher values in comparison with the straight channel, which is due to intense mixing in the chaotic geometry, such that the velocity and temperature contours in the C-shaped channel are more uniform than those in the straight one. Using the two-phase method, the concentration distribution is obtained non-uniform at the cross section of the straight channel, while intense mixing in the C-shaped channel makes distribution of the nanoparticles uniform. In comparison with water, using the nanofluid through both channels presents higher heat transfer and pressure drop. However, merit of using the nanofluid in the C-shaped channel is greater than that in the straight one. In this regard, simultaneous application of nanofluids, as heat transfer fluids, and chaotic channel, as a modified geometry, can result in not only higher energy efficiency, but also preventing nanoparticles agglomeration due to the intense mixing. (C) 2014 Elsevier Ltd. All rights reserved.