Experiments were conducted to investigate the single phase convective heat transfer in a compact heat sink consisting of 26 rectangular microchannels of 300 mu m width and 800 mu m depth. The relative roughness is estimated to be 4-6%. Deionized water was used as the working fluid. Tests were performed with the Reynolds number range of 162-1257, the inlet liquid temperatures of 30, 50 and 70 degrees C and the heating powers of 140-450 W. The planform area is 5.0 x 1.53 cm(2). it is found that the friction factors and local and average Nusselt numbers significantly depart from those of conventional theories, possibly attributable to the surface roughness. The hydraulically developed but thermally developing flow results in decreased Nusselt numbers versus the non-dimensional axial distance. The temperature dependent fluid physical properties also influence the heat transfer characteristics to some extent. Correlations were provided for the friction factors and the Nusselt numbers. (c) 2005 Elsevier Ltd. All rights reserved.