An experiment has been conducted to measure the friction factor of laminar flow of deionized water in smooth silicon microchannels of trapezoidal cross-section with hydraulic diameters in the range of 25.9-291.0 mum. It is shown that the friction constant of these microchannels is greatly influenced by the cross-sectional aspect ratio, W-b/W-t. Based on the 334 data points, a correlation equation for the friction constant of a fully developed laminar flow of water in these microchannels is obtained in terms of the cross-sectional aspect ratio. The experimental data is found to be in good agreement with an existing analytical solution for an incompressible, fully developed, laminar flow under no-slip boundary condition. It is confirmed that the Navier-Stokes equations, are still valid for the laminar flow of deionized water in smooth silicon microchannels having hydraulic diameter as small as 25.9 mum. For smooth channels with larger hydraulic diameters of 103.4-291.0 mum, transition from laminar to turbulent flow is found at Re = 1500-2000. (C) 2003 Elsevier Science Ltd. All rights reserved.