This article has reported the results of rheological testing of low-density polyethylene (LDPE) and its calcium carbonate composites containing 7, 14, 21, and 28 wt% filler, respectively. The polymer composites were produced in a twin-screw extrusion process. The assessment of the rheological properties of the polymeric materials was made under extrusion process conditions, using an in-line rheometer with an extrusion slit die (W = 20, H = 2, L = 150 mm), at temperatures of 170 degrees C, 180 degrees C, and 190 degrees C, respectively. The rheological parameters were determined based on the Ostwald-de-Waele power law model. The employed testing stand enabled the assessment of the effect of filler addition and slit die temperature on the variations in viscosity, power law index (n), consistency index (K), maximum flow velocity (V-max), and maximum flow profiles (V-z), under the conditions of technological processing (extrusion) of plastics. POLYM. ENG. SCI., 59:E16-E24, 2019. (c) 2018 Society of Plastics Engineers