Polypropylene/silica (PP/Silica) composites were prepared by two steps: tetraethyl ortho-silicate (TEOS) was first impregnated into PP matrix using supercritical carbon dioxide (SC CO2) as a swelling agent and carrier followed by hydrolysis/condensation reaction of TEOS confined in polymer network. The effects of soaking temperature, CO2 density, and initial TEOS concentration on the mass uptake of TEOS in PP were investigated. Hybrids with high silica load could be achieved by repeating the steps. The hybrids were characterized by attenuated total reflectance infrared (ATR-IR), Si-29 solid-state nuclear magnetic resonance spectroscopy (Si-29 solid-state NMR), thermal gravimetric analysis (TGA), transmission electron microscope (TEM), scanning electron microscopy (SEM), and mechanical testing. The results showed that nanosized silica networks were formed and distributed uniformly in PP matrix, and some polymer molecules might entangle with the silica networks. The PP/Silica composites had much higher tensile strength and Young's modulus than virgin PP due to their special morphology.