The heterogeneous free-radical polymerization of styrene within supercritical carbon dioxide swollen silicon rubber film has been conducted to prepare the silicon rubber/polystyrene (SR/PS) polymer blends. The PS content in the blends can be controlled by adjusting the soaking time and the concentration of styrene in the supercritical fluid. Scanning electron microscopy (SEM) indicates that the PS phase is uniformly distributed in the blends and the phase size is very small (100 nm to 300 nm), although the two polymers are very different and incompatible. The mechanical properties of the blends and the average molecular weight of the PS polymerized in the matrix were also measured. The results indicate that the average molecular weight of the PS in the blend depends on the PS content or its phase size. The tensile strength of the blends is higher than that of original SR substrate, and there is a maximum in tensile strength vs. PS content. The Young's modulus of the blends increases monotonously with PS content in the blends.