The presence of the electrical double layer near a solid-liquid interface results in the electro-viscous effect on pressure-driven liquid how through microchannels. The objective of this paper is to examine the magnitude of the additional flow resistance caused by the electrokinetic effect in microchannels. Deionized ultrafiltered water, 10(-4) and 10(-2) M aqueous KCl solutions, 10(-4) M AlCl3 solution, and 10(-4) M LiCl solution were used as the testing liquids. Carefully designed how 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, the 10(-4) M KCI solution, and the 10(-4) M LiCl solution was found to be significantly higher than the prediction of the conventional laminar flow theory at the same Reynolds number Such a high how resistance and the resulting high apparent viscosity strongly depend on the channel's height, the ionic valence, and the concentration of the liquids. The zeta potentials for the liquid-solid systems were calculated by using the measured streaming potential data. The experimentally determined dP/dx similar to Re relationships were compared with the predictions of a theoretical electro-viscous how model, and a good agreement was found for pure water, 10(-4) M KCl solution, and 10(-4) M AlCl3 solution systems. The present electrokinetic how model cannot interpret the flow characteristics of the LiCl solution.