Modern heat rejection systems, such as micro-heat sinks, are attractive because of their potential for high performance at small size and low weight. However, the impact of microscale effects on heat transfer have to be considered and quantitatively analyzed in order to gain physical insight and accurate Nusselt number data. The relative surface roughness (SR) was selected as a key microscale parameter, represented by a porous medium layer (PML) model. Assuming steady laminar fully developed liquid flow in microchannels and microtubes, the SR effects in terms of PML thermal conductivity ratio and Darcy number on the dimensionless temperature profile and Nusselt number were analyzed. In summary, the PML characteristics, especially the SR-number and conductivity ratio k(m)/k(f), greatly affect the heat transfer performance where the Nusselt number can be either higher or lower than the conventional value. The PML influence is less pronounced in microtubes than in parallel-plate microchannels. (c) 2005 Elsevier Ltd. All rights reserved.