Heat transfer characteristics of gamma-Al2O3/water and TiO2/water nanofluids were measured in a shell and tube heat exchanger under turbulent flow condition. The effects of Peclet number, volume concentration of suspended nanoparticles, and particle type on the heat characteristics were investigated. Based on the results, adding of naoparticles to the base fluid causes the significant enhancement of heat transfer characteristics. For both nanofluids, two different optimum nanoparticle concentrations exist. Comparison of the heat transfer behavior of two nanofluids indicates that at a certain Peclet number, heat transfer characteristics of TiO2/water nanofluid at its optimum nanoparticle concentration are greater than those of gamma-Al2O3/water nanofluid while gamma-Al2O3/water nanofluid possesses better heat transfer behavior at higher nanoparticle concentrations. (C) 2009 Elsevier Ltd. All rights reserved.