Antibacterial polymers such as polymeric phosphonium salt were successfully grafted onto nano-sized silica surface by the treatment of poly(p-styrene sodiume sulfate)-grafted silica with alkyl phosphonium chloride. By the grafting of polymeric phosphonium salt onto silica surface, the silica surface shows extremely hydrophobic nature, based on the alkyl chains. The antibacterial polymer-grafted silica dispersed in poly(dimethylsiloxane). It was found that the surface of the obtained composite shows high antibacterial activity against S. aureus. In addition, the grafted silica with polymeric quaternary ammonium salts was also synthesized in order to estimate antibacterial activity by the method.