The objective of this study was to investigate the effect of supercritical carbon dioxide (scCO(2)) during continuous extrusion on controlling phase structure of polypropylene (PP)/polystyrene (PS) blends and the corresponding mechanical properties and foamability. Viscosity reduction of PP and PS was studied using a slit die rheometer attached to a tandem extrusion system. The scCO(2) was injected at 2.0, 4.0 and 6.0 wt% to PP/PS blends during extrusion. It was found that a sharp decrease in the size of the dispersed phase was achieved with the injection of scCO(2). The size of the minor phase for 75/25 PP-2/PS blend with 4.0 wt% scCO(2) was significantly reduced to 50 nm, due to the viscosity ratio of PP-2 to PS close to one. The mechanical properties of the polymer blends were closely related to the phase structure, and better dispersion favored enhanced mechanical properties. Foamability of the PP/PS blends was also found to be closely dependent on phase morphology, and better dispersion endorsed uniform cell structure with smaller cell size and high cell densities.