Impregnation of low-density palyethylene (LDPE) with styrene using supercritical (SC) CO2 as the swelling agent was studied at 35 degreesC in a pressure range from 90 to 160 bar. The concentration of styrene in the fluid phase ranged from 0 to 1.668 mol/L. The soaking time varied from 4 to 36 h. In the presence of azobis(isobutyronitrile), the initiator, styrene polymerized to some extent in the soaking process, which results in an increase in the mass uptake. LDPE/polystyrene (PS) composites were prepared by the polymerization of styrene further in SC CO2-swollen LDPE substrates at higher temperature. Using this method, the composition of the composites can be controlled by the soaking time, pressure, and styrene concentration in the fluid phase. Some of the PS molecules in LDPE/PS blends entangle with the polyethylene molecules, which causes significant improvement in the impact strength, the tensile strength, and the elongation at the break.