Supercritical carbon dioxide (CO2) was used in extrusion for the improvement of polymer blend properties. Various extruder configurations using a twin-screw extruder and a single-screw extruder were designed for investigating the effects of dissolved supercritical CO2 on the viscosity and morphological properties of polyethylene, polystyrene, and their blends. The viscosities of the polymer/CO2 and the blend/CO2 solutions were measured at various concentrations of CO2 rind PE/PS blending ratios using a wedge die mounted on the twin-screw extruder. The effect of CO2 on the morphology of PE/PS blends was also investigated using a twin/single-screw tandem system. This system allowed for preferential dissolution of the CO2 into the matrix and/or dispersed polymer phase. By introducing devolatilization to the tandem system, the morphological behaviors of PE/PS blends were investigated on unfoamed filaments. In general, the mixing between two polymers was improved by the dissolution of CO2. The size reduction of the dispersed phase was explained using the viscosity ratio of the two polymers. Finally, the interface between foamed and unfoamed polymers was studied in a bilayer structure produced by using a special configuration of the tandem system.