It has been demonstrated that the pore size and surface charge density of microfiltration (MF) membranes can be determined from the streaming potential (SP) measured over a wide range of KCl concentrations using a microscopic model for the SP. The SP was measured for the MF membranes made of various materials and with various pore sizes. The absolute value of zeta(obs), which was obtained using the Helmholtz-Smoluchowski (HS) equation, reached a maximum value with increasing KCl concentration and could be assigned to a theoretical zeta potential, zeta(theory), at higher KCl concentrations. The SP versus conductivity plots showed unique curves reflecting changes in both the surface charge density and the behavior of the electric double layers (EDLs) in the pore, which will overlap at lower conductivity but be compressed at higher conductivity. The pore size and surface charge density were simultaneously determined by the analysis of this change in SP using the microscopic model for SP. In polycarbonate membranes the pore size obtained by this analysis showed a good linear relationship with the nominal pore size, which is almost equal to the morphological pore size determined by SEM image analysis, although the absolute value of the pore size obtained by this analysis was lower than the nominal pore size.