Crude silica was modified with silsisquioxane (SSO), which was originated from vinyltriethoxysilane, leading to modified silica. The modified silica with vinyl groups was employed to reinforce polydimethylsiloxane (PDMS) through participating in the hydrosilation reaction. Inorganic-organic hybrid network was formed to obtain higher mechanical properties. Fourier transformation infrared spectroscopy (FTIR) confirmed that the vinyl groups were bonded to the surface of silica. Transmission electron microscopy (TEM) showed that the diameter of the modified silica increased and the shape became irregular. The effects of the amount of SSO, the loading of modified silica, and the vinyl/hydrogen mole ratio on the mechanical properties of the cured PDMS system were investigated. It was found that 40 parts of SSO could result in a sufficient surface modification of the silica. The tensile strength and the modulus (reflected by the stress at 100% elongation) kept increasing with increasing loading of silica. When the vinyl/hydrogen mole ratio was 1:1, the most effective reinforcement was obtained.