Nanometer dispersed polypropylene/polystyrene (PP/PS) interpenetrating networks (IPNs) have been prepared by the radical polymerization and crosslinking of styrene (St) within supercritical (SC) CO2-swollen PP substrates. In this method, monomer St, crosslinking agent divinyl benzene (DVB), and the initiator benzoyl peroxide were first impregnated into PP matrix using SC CO2 as a solvent and swelling agent at 35.0 degreesC, and then the polymerization and crosslinking were carried out at 120 degreesC. The composition of the IPNs can be controlled by SC CO2 pressure, concentrations of St and DVB in the fluid phase. Transmission electron microscopy shows that the PS is homogeneously dispersed in the IPNs and its phase size is in the range of 20-30 nm. The impact strength, tensile strength, and elongation-at-break of the PP/PS IPNs increase with increasing PS percentage in the IPNs.