Surface roughness may have a significant impact on microchannel performances, since at such a small scale it is nearly impossible to obtain an actual smooth surface. The numerical approach allows a detailed description of the surface imperfections; thus, we can easily separate roughness from other microscale effects. In this paper, roughness is modelled as a set of three-dimensional conical peaks distributed on the ideal smooth surfaces of a plane microchannel. Different peak heights and different peak arrangements are considered at various Reynolds numbers. Periodicity conditions in both transverse and streamwise directions allow the reduction of the domain to a small volume containing one or two peaks. The performances of parallel plate rough channels are compared with standard correlation. Results show a remarkable effect of roughness on pressure drop, and a weaker one on the Nusselt number. The performances are dependent on the geometrical details of the roughness elements. The impact of the uncertainty in the definition and measurement of the hydraulic diameter is also discussed. (C) 2007 Elsevier Ltd. All rights reserved.