An unmodified and stearic acid-modified CaCO3 were added sep. to polypropylene (PP) in concns. 5-30% by mass and then extruded in a twin-screw extruder. Courses of mass flow rate index (MFR) were detd. by using a plastometer (off-line measurements) while courses of viscosity curves were detd. by using a capillary rheometer (off-line measurements) and extrusion rheometer (in-line measurements). The addn. of modified CaCO3 resulted in increasing the MFR value of composites and in decreasing their viscosity for all tested concns. of CaCO3. The trends were also obsd. for the unmodified CaCO3 but only for concns. up to 10%.