The rheological properties of synthetic hectorite/water suspensions have been measured by a cone-plate rheometer as a function of their pH values. Synthetic hectorite was dialyzed to remove electrolyte impurities involved, and then utilized for preparing suspensions. When HNO₃ and NaOH were added to the aqueous suspensions to adjust pH values, the yield value and shear modulus of the suspensions decreased both in acidic and alkali regions. It was found by the use of a Cryo-SEM observation that different types of coagulated structures existed for different pH regions. These results were probably caused a change in the charge at the edge of hectorite particles as follows: (1) in the acid region, the electrostatic attraction between negatively charged faces and positively charged edges of the particle are dominant and a compact card-house structure is formed; (2) in the alkali regions, the edges of the particle are negatively charged and resulting electrostatic repulsion leads to dispersed state of the particle; (3) in the neutral region, around the isoelectric point of the particle's edge an extended network structure is formed due to edge-to-edge association.