zeta potentials of synthetic smectites with different locations of lattice charges, i.e., saponite and hectorite, were investigated as functions of pH and NaCl concentration by a flat plate streaming potential technique where two smectite-coated plates were separated by a Teflon gasket to form a slit channel. The lattice charge of saponite originates from the isomorphous substitution of Si4+ by Al3+ ions in tetrahedral silica sheets, whereas the lattice charge of hectorite is caused by the isomorphous substitution of Mg2+ by Li+ ions in octahedral magnesia sheets. The zeta potential for saponite was more sensitive to the pH of electrolyte solution than that for hectorite in acidic solution. Such a difference in pH dependence of the zeta potential could be explained by the difference in the location of the lattice charges between saponite and hectorite as a result of an analysis of the inner Helmholtz layer capacitance by using the Gouy-Chapman-Stern-Graham (GCSG) model with a single-site dissociation of surface groups. On the other hand, the dependence of the zeta potential on NaCl concentrations could not be fully interpreted by the GCSG model with a single value of the outer Helmholtz layer capacitance, which suggests a shift of the shear plane and/or a change in the dielectric constant of the outer Helmholtz layer with changing NaCl concentration.