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Background: Dental composite resins containing Nano-Particles (NPs) of Zinc-Oxide (ZnO) are reported to exhibit antibacterial properties against cariogenic bacteria; but the important physical and mechanical properties of such materials need more studies. The aim was to evaluate the antibacterial, and some of the important physical and mechanical properties of composite resin containing ZnONPs. Methods: 0.5% and 1% weight NPs of ZnO were mixed with flowable composite. The composite resins were tested for the antibacterial properties against MutansStreptococcus (MS) and Lactobacillus (L), flexural strength, micro-hardness, degree of conversion, and water sorption. Data for direct contact test were analyzed by two-way ANOVA and Tukey post HOC tests. Data of water sorption were analyzed by Kruskal-Wallis and Mann-Whitney tests. Data analyses for other tests wereOne-way ANOVA and Dunnet tests. Significance level was set at 0.05. Results: Incorporation of NPs decreased the number of MS and L counts. NPs added to composite resin caused a reduction in all the physical and mechanical properties (p<0.05) except water sorption (p>0.05). Flexural strength, micro-hardness, and degree of conversion were significantly lower in composite resin with 1% NPs compared to the composite with 0.5% NPs, while 1% NPs indicated higher antibacterial properties. Conclusions: Although incorporation of ZnO NPs into flowable composite imparted antibacterial properties, it reduced some physical and mechanical properties of composite resin. These properties were lower in composite containing 1% ZnO NP, compared to 0.5% NP. Water sorption of the composite resin did not change significantly by addition of ZnO NPs.