It is typically assumed that flow field has very little or no effect on the electric double layer distribution. Here, we study this effect by means of a dilute electrolyte (deionized water) in a streaming potential mode for a 40-mum parallel-plate microchannel. By measuring the electrical potential downstream along the channel surface, we have shown that the potential distribution along a streamwise direction can be nonlinear. By means of a current continuity equation, we also presented numerical simulation results of potential distribution for pressure-driven flow with electrokinetic effects. The simulated results agree well with those from experiment. Results indicate that the conduction current in the streamwise direction is only a fraction of the streaming current. As a result, the zeta-potential values determined from streaming potential measurements are generally smaller than those from streaming current measurements. It is expected that this discrepancy would be smaller for a more concentrated electrolyte.