The viscosity of highly concentrated suspension in liquid prepolymer (HTPB) depends oil the concentration and the distribution of the particle size. The relative viscosities of multimodal system are calculated and compared to the results obtained from the monomodal suspension. In order to get an ideal minimum viscosity of the suspension paste, each group of the particle should be monodispersed. The weight fraction of the lowest porosity, for bimodal, coincides with that of the minimum viscosity. Since the viscosity of the mixed system depends on the specific surface area of particles, their shapes should be spherical. The theoretically calculated minimum viscosity of a multimodal system coincides with the viscosity obtained experimentally when the particle size ratios are larger than 10.