The presence of the electrical double layer near a solid-liquid interface results in the electro-viscous effect on pressure-driven liquid flow through microchannels. The objective of this paper is to examine the magnitude of the additional flow resistance caused by the electrokinetic effect in microchannels. De-ionized ultra-filtered water and aqueous KCl solutions of two different concentrations (10(-4) and 10(-2) M) were used as the testing liquids. Carefully designed flow measurements were conducted in three silicon microchannels with a height of 14.1, 28.2 and 40.5 mum, respectively. The measured dP/dx for the pure water and the low concentration solution were found to be significantly higher than that without electro-viscous effect at the same Reynolds number. Such a difference strongly depends on the channel's height and the ionic concentration of the liquids. The how-induced streaming potential was also measured and the data confirm a higher electro-viscous effect on the how of pure water and low concentration KCl solution. The experimental results were compared with the predictions of a theoretical electro-viscous flow model, and a good agreement was found.