The rheological characteristics of synthetic hectorite/water suspensions have been measured by a cone-plate rheometer as a function of additive NaCl concentration (C NaCl ). The hectorite particles, dialyzed preliminarily to remove electrolytic impurities involved, were utilized for preparing a 2.0 wt% aqueous suspension. The apparent viscosities of the suspensions decreased monotonically with increasing C NaCl ; these changes in the viscosities came from the primary electroviscous effect. In dynamic viscoelastic measurements of the suspensions, we found that the critical strain also decreased with increasing C NaCl . By the use of a Cryo-SEM, we observed that different types of the particle-coagulated structures in the various suspensions with respective NaCl concentrations. These findings can be explained by the different interaction modes originated from the electrostatic charge near the surface of hectorite particles, as in the following situations : (1) in the low C NaCl region, the electrostatic repulsion among the particles leads effectively to form an expanded network structure in the system; (2) in the high C NaCl region, the van der Waals attraction force among the particles governs the interaction mode in the system, thus resulting to form a dense-packed coagulation structures.