INTRODUCTION: During the past decade, composites have become the most commonly used restorative materials. The rate of dental caries following treatment with composite resin is high. Therefore, one of the most applicable methods for preventing enamel demineralization around the restorations is using dental materials resistant to the bacterial accumulation OBJECTIVES: To synthesis and characterize antibacterial nanoparticles and to evaluate the effect of blending microhybrid composite with zinc oxide nanoparticles (ZnO), Chitosan (Cs) and combination of both Chitosan/Zinc oxide nanoparticles (Cs/ZnO) and properties of these nanoparticles on the composite resin. MATERIALS AND METHODS: Three antibacterial nanoparticles were prepared and characterized in terms of particle size, zeta potential, shape, morphology and functional group determination. Minimum inhibitory concentration of the nanoparticles was determined. The nanoparticles were incorporated into commercial microhybrid composite resin. The antibacterial properties against Streptococcus mutans were evaluated by disc diffusion test and direct contact test. The results were analyzed using ANOVA test at p ≤ .05 significance level. RESULTS: For agar diffusion disc, incorporation of ZnO nanoparticles into the composite resin results in an antibacterial effect which lasted for up to 12 weeks, while for the Cs and Cs/ZnO nanoparticles the antibacterial effect lasted for up to 2 weeks. The direct contact test visualized under SEM also showed that incorporation of ZnO nanoparticles into composite resin to be the most inhibitory in all the 4 groups, denoting that ZnO-NPs has a far better inhibitory effect than Cs-NPs and Cs/ ZnO-NPs. CONCLUSIONS Antibacterial nanoparticles could be synthesized and characterized by Zetasizer NanoZS, scanning electron, transmission microscopy and Fourier transform infrared spectroscopy. Incorporation of ZnO, Cs and Cs/ZnO nanoparticles into the composite resin could significantly inhibit the S. mutans. The antimicrobial efficacy of the ZnO nanoparticles blended with microhybrid composite resin was confirmed for a duration up to 12 weeks