In polymeric foaming processes using CO2 as a physical foaming agent, polymer viscosity is one of key factors determining the cell size of the foams. The polymer viscosity is reduced when the foaming agent (CO2) gas is dissolved into the polymer. It is necessary to know a quantitative relationship between dissolved gas concentration and viscosity reduction, in order to control of cell size precisely and density in foaming processes. In this study, the relationship between shear viscosity of polypropylene and concentration of CO2 dissolved in the polymer was examined, by use of a capillary rheometer equipped at exit of extruder. The CO2-induced viscosity reduction can be predicted by employing the free volume theory and PVT equation of polymer-CO2 mixture. The estimations given by the developed model showed good agreement with the measured data.