The rheological behavior and microstructure of bimodal suspensions of core-shell structured swollen particles have been examined with changing volume ratio of two different sized particles. As the volume fraction of large particles increases, the viscosity, degree of shear-thinning, and the critical shear stress sigma(c) decreases, while the interparticle distance of the microstructure increases. The suspensions exhibit single mode rheological behavior and have a single diffraction peak in the SAXS profiles. These results suggest that the bimodal suspensions of the core-shell structured swollen particles behave likely to unimodal suspensions of hard spheres with alloy like single mode microstructure composed of hypothetical intermediate size particle. The relationship between sigma(c) and xi can be represented as sigma(c) = 3kT/4 pi xi(3), which corresponds to the dynamics of the Brownian hard sphere model with being the particle diameter. These findings indicate that the shear-thinning of the suspensions can be attributed to dynamical competition between the thermal motion and the hydrodynamic motion under shear flow and that the mechanism can be applied to bimodal suspensions of the swollen particles as well as unimodal suspensions of hard spheres. (c) 2006 Wiley Periodicals, Inc.