Abstract: The effects of the volume fraction of solid particles, φ, the ratio of particle diameters, R, and the volume fraction of small particles in solid amount, φ_s, on rheological properties in concentrated suspensions were studied. Commercial ammonium perchlorate and aluminium powder were used as dispersed phase, and a polyether diol as continuous phase. In bimodal dispersed systems the viscosity of the systems is minimum at φ_s=0.30, and decreases as R decreases. The influence of small particles is shown to be very important in the pseudoplastic behavior of concentrated suspensions. With the increase of small particle content or the decrease of particle diameter ratio in suspension, the pseudoplastio extent of the system increases. The effects of φ and φ_s on system become stronger when non-spherical particles exist in suspension. In bimodal system with wide distribution of spherical particle size the effect of φ on the pseudoplasticity of the system is less than that in the system with narrow distribution.