---

Abstract Background and Objective: the plants with the less side effects and drug resistance have been used worldwide. In this study, we compared the inhibitory effects of nanocopper and essential oil of Melissa officinalisL.a on E.coli and S.mutans in In Vitro condition. Material and Methods: for studying diameter of inhibitory zone, disk agar diffusion method was used. Then, antibacterial effects of these substances were evaluated by treating them for 24 hours in Nanocopper particles (concentration of 100 and 500 ppm) and essential oil (12.5% to 100%), and analyzed by ANOVA. Results: twenty-four hours after treatment, nanocopper had no inhibitory effect on these bacteria. However, the diameter of inhibitory zone for E.coli and S.mutans was 31.30±0.13mm and 16.30±0.13mm, respectively. There was not any synergistic effect between different concentrations of this plant and 500ppm of nanocopper after 24 hours of treatment on E.coli. But for S. Mutans, the diameter of inhibitory zone for mixture of 50% & 25% essential oil and 500ppm concentration of nanocupper was increased significantly compared to only essential oil (p=0.001,p=0.01). Conclusion: based on the findings, nanocopper particles and essential oil of Melissa officinalis L.a have not any synergic effects on E.coli, but with some concentrations of this plant the reverse is true on S.mutans. Keywords: Essential Oil of Melissa Officinalis, Nanocopper Particles, E.Coli, S.Mutans