The numerical modeling of the conjugate heat transfer and fluid flow of Al2O3/water nanofluid through the microchannel heat sink is presented in the paper. The laminar flow regime was considered along with viscous dissipation effect. The microchannel heat sink with square microchannels and D-h = 50 mu m is considered. The heat flux was fixed to q = 35 W/m(2) with heating and cooling cases. The water based Al2O3 nanofluid was encountered with various volume concentrations of Al2O3 particles phi = 1-9% and three diameters of the particle d(p) = 13, 28 and 47 nm. The analysis is performed on the results obtained for the local heat transfer coefficients based on a fixed pumping power. The results reveal a different local heat transfer behavior compared to the analysis made on a basis of the constant Re. (C) 2011 Elsevier Ltd. All rights reserved.