For most microchannels made of hydrophobic materials such as polymers, velocity slip occurs at the wall, affecting volumetric flow rate of electroosmotic flow Qeof and streaming potential (??str?z). Since most techniques exploit Qeofor (??str?z) to determine the zeta potential, ?, it is very difficult to measure ? of hydrophobic walls, if the slip coefficient b is not found a priori. Until now, Qeof and (??str?z) are known to depend on ? and b in a same functional form, which makes it impossible to estimate ? or b separately using measurements of Qeof and (??str?z). However, exploiting the analytic formula for Qeof and (??str?z) derived in the present work, it is found that the effect of ? and that of b on Qeof and (??str?z) can be separated from each other by varying the bulk ionic concentration. Thus, the slip coefficient as well as the zeta potential of hydrophobic microchannels can be found with reasonable accuracy by means of a nonlinear curve fitting method using measured data of Qeof and (??str?z) at various bulk ionic concentrations. The present method allows an accurate estimation of slip coefficient of hydrophobic microchannels, which is quite simple and cheap compared with methods employing microparticle velocimetry.