Supercritical carbon dioxide (scCO2) has advantages of being incorporated in polymer with high solubility and of being recovered easily by depressurizing. ScCO2 reduces the viscosity of polymer melt and it is expected to be use as a plasticizing agent. In this work, we studied on the effect of scCO2 on the rheological properties of polymer melts during extrusion process. Slit die attached to twin screw extruder was used to measure the viscosity of polymer melts plasticized by supercritical CO2. A gas injection system was devised to accurately meter the supercritical CO2 into the extruder barrel. Measurements of pressure drop within the die, confirmed the presence of a one phase mixture and a fully developed flow during the measurements. The viscosity measurement of polypropylene was performed at experimental conditions of various temperatures, pressures and CO2 concentrations. We observed that melt viscosity of polymer was dramatically reduced by CO2 addition.