The bimodal system of simple cubic particle configuration was studied as an extension of the earlier works. In this model two different size rigid spheres were arranged alternatively. The diameter ratio of the two particles ranged from 0.05 to 1.0 and the total particle concentration by volume ranged from 52.4 to 79.2 per cent. It was theoretically shown that the relative viscosity of the model system decreased, in comparison to the unimodal system, as the particle size ratio decreased at a given concentration, whereas the change relative viscosity wad insignificant below the limiting size ratio of about one tenth and the viscosity of the bimodal system was almost equivalent to that of the unimodal system above the size ratio of about eight tenth. These facts were in good agreement with the experimental data reported in the literature.