CaCO3 was modified with stearic acid (up to 3.5% by mass) and used for filling polyethylene and polypropylene-matrix composites (64%). The composites were studied for torque during homogenization, mass flow rate and viscosity at 230 degrees C. The shear thinning behavior of the molten composites was obsd. Their rheol. properties were improved by the increased slippage on the polymer-filler interface.