 Composite resin materials are widely used in the dental clinic for replacement of hard tissues. Although the mechanical properties and wear resistance of these materials have been improved substantially, their antibacterial properties are still limited. These resin-based materials accumulate more dental plaque than other restorative materials both in vitro, and in vivo, which may result in secondary caries. A number of reports described experiments in which composite resins were impregnated with antibacterial agents such as antibiotics, silver ions, iodine and quaternary ammonium compounds, and gradually released them. However, release of antibacterial agents into the surrounding milieu at various releasing rates had several disadvantages: a decrease in the mechanical properties of the carrier material over time, short-term effectiveness, and possible toxicity if the release is not properly controlled. As compared with conventional antibacterial agents of low molecular weight, the advantage of polymeric antibacterial agents is that they are nonvolatile, chemically stable, can be chemically bound within the polymer carrier via active groups for improved integration in the composite, and are difficult to penetrate through the skin. It has been reported that polycations exhibit antibacterial properties, i.e. interact with and disrupt bacterial cell membranes. A number of polymers with antibacterial properties were developed for this purpose, including soluble and insoluble pyridinium-type polymers involved in surface coating. Several reports have described incorporation of a methacryloyloxydodecylpydidinium bromide MDPB monomer in composite resins that showed no release of the incorporated monomer but still exhibited antibacterial properties. The objective of this study is to evaluate clinically the safety and efficacy of alkylated polyethylenimine PEI nanoparticles in composite resin restorative materials . In an in vitro study, addition of a small percent 1 ww of nanoparticles did not affect significantly the flexural strength of the commercial materials. The mechanical properties of the new composites were close to those of the original composite, but exerted a strong antibacterial activity upon contact that lasted for at least six months. 