This study studied the effect of filler addition on the viscosity of polymeric melts. Three types for fillers – precipitated calcium carbonate (Socal U1), precipitated calcium carbonate pretreated with stearic acid (Socal U1S2), and titanium dioxide (P25) – were added to the polyolefins (polypropylene and high-density polyethylene). The fillers were added in three different mass fractions and their melt flow rate (MFR) was observed and compared to the MFR of polymer granules and of the reference samples at different temperatures. Determining the MFR values helped determine the viscosity of the tested samples since the two values are reversely proportional. Main focus of the study was observing the change in viscosity of the polymeric components as the mass fraction of the different fillers was increased at three temperatures. The results confirmed that increased temperature decreases the viscosity of the polymeric melt. Different fillers had different effects on the viscosity of the polymeric melt. Greatest increase in viscosity with increased filler mass fraction was observed in the polyethylene composite with TiO2 P25 filler. The smallest difference in the change of viscosity of both polyolefin composites in comparison to the reference samples was observed after the addition of CaCO3 Socal U1S2 filler treated with stearic acid.