The viscosity of aqueous montmorillonite suspensions is affected by many factors, most notable among them include the soluble salt content and composition of exchangeable cations. However, difference in the viscosity of these suspensions remained even at almost identical soluble salt and exchangeable cation compositions. Because of these, the authors made this research based on the following hypothesis: the determining factor for the viscosity of aqueous montmorillonite suspensions is the aspect ratios of montmorillonite particles. Eight montmorillonite samples from Yamagata-Japan, Wyoming-USA and Gujart-India, having wide range of viscosity and aspect ratio, were used. In the experiments, the de-carbonated and Na-exchanged samples in addition to the untreated samples, both purified from bentonite suspension, were utilized. Surface area of basal planes and heights of 100 montmorillonite particles randomly-selected from the untreated samples were measured using AFM. The surface area of the basal planes was converted to the diameter of an equivalent circle. The aspect ratio was then calculated by dividing the diameter of the obtained equivalent circle with the height of the particle. The calculated values of the aspect ratio were at 260-380, 190-210 and 129 for the montmorillonites sampled from Yamagata, Wyoming and Gujarat, respectively. The aspect ratios of some Na-exchanged montmorillonite samples were also determined from the viscosity of diluted suspensions. The values of the aspect ratio obtained using this different method showed good agreement with the results by AFM measurement. This means that montmorillonite particles are suspended in water without folding or bending. For Na-exchanged samples, aspect ratio of montmorillonite particles and viscosity of aqueous suspensions showed good correlation (R^2=0.96). Using Simha's and Einstein's equations, the reason why the aspect ratio is the determining factor for the viscosity of aqueous montmorillonite suspensions could be explained as follows: in the aqueous suspensions, montmorillonie particles are rotating with Brownian movement of water molecules and occupying an apparent volume of sphere having diameter equivalent to the aspect ratio (nm). Apparent volume fraction of montmorillonite, which is calculated by multiplying the apparent volume of the montmorillonite particle and the number of particles in a 1ml suspension, is controlled by the aspect ratio. Because the aspect ratio affects numerically on the order of cube power to the apparent volume fraction of montmorillonite, while the number of particles in suspension affects only on the order of single power, in consideration of Einstein's equation. Montmorillonite that have large aspect ratios even with less number of particles will occupy a large apparent volume fraction in suspension and will have high viscosity. In contrast, montmorillonite with a smaller aspect ratio and large number of particles will occupy a smaller apparent volume fraction and the suspension will have low viscosity. Thus, the determining factor for the viscosity of aqueous montmorillonite suspensions is the aspect ratio of montmorillonite particles. Because this study was made using 8 montmorillonite samples having wide range of aspect ratio and viscosity, we believe that the conclusion obtained is universal.