This paper analytically examines the effects of adding nanoparticles on the entropy generation of water-Al2O3 and ethylene glycol-Al2O3 nanofluid flows through a circular pipe under uniform wall heat flux thermal boundary condition in both laminar and turbulent regimes. Approved formulations of mixtures are used for density and specific heat of the nanofluids, and a model developed by Maiga et al. [9] based on experimental studies is used for viscosity and conductivity of the nanofluids. It is found that adding nanoparticles improves the thermal performance of water-Al2O3 flow with Re numbers less than 40,000 and ethylene glycol-Al2O3 flow with Re < 11. It is shown that adding nanoparticles leads to increased entropy generation in the cases that fluid flow (pressure drop) irreversibility is dominant. Moreover optimum conditions (based on the entropy generation sense) for both laminar and turbulent nanofluid flows are obtained. (C) 2010 Elsevier Ltd. All rights reserved.