This study investigates the design, construction and instrumentation of an experimental microchannel, with a rectangular cross-section and large aspect ratio, that allows characterization of the flow and convective heat transfer under well defined and precise conditions and makes it possible to vary the hydraulic diameter of the microchannel. The flow friction coefficient is estimated by direct pressure drop measurements inside the microchannel in a zone where the flow is fully developed. Since the wall thermal conditions inside the microchannel can not be measured directly, their estimation requires temperature measurements in the wall thickness and an inverse heat conduction method. The thermal and hydrodynamic results obtained by varying the hydraulic diameter between 1 mm and 100 mu m do not deviate from the theory or empirical correlations for large-scale channels, These results let us confirm that for smooth walls the continuum mechanics laws for convection and fluid mechanics remain valid in microchannels of hydraulic diameter greater than or equal to 100 mu m. (C) 2008 Elsevier Ltd. All rights reserved.