Suspensions of six samples of calcium carbonate (CaCO₃) with different average particle sizes in 1.5 wt% aqueous solution of hydroxyethy1 cellulose (HEC) were prepared. The effect of filler particle size on such rheological properties as viscosity and yield value was investigated. The smaller CaCO₃ particles exhibited the greater tendency of particle agglomeration. Unlike the aqueous HEC solution which shows Newtonian flow region, suspension systems exhibited a yield behavior. At the same filler concentration, suspension including smaller CaCO₃ particles gave higher viscosity and greater yield value than that including larger CaCO₃ particles. Further the value of yield stress was increased with in creasing CaCO₃ concentration. The Mooney equation gave a qualitive explanation of the effect of particle size of CaCO₃ on the suspension viscosity.