Objectives: Tooth decay is an irreversible complication of calcified tooth tissues that is characterized by demineralization of the inorganic part and destruction of organic matter in the tooth and often leads to the formation of cavities. In the present study, we investigated the antibacterial effect of zinc nanoparticles (ZnNPs) green synthesized with Lavandula vera DC. by microwave method on Streptococcus mutans and Actinomyces viscosus compared with chlorhexidine. Material and Methods: ZnNPs were green synthesized using L. vera extract using the microwave and were characterized by scanning electron microscope, X-ray diffraction technique, nano-sizer-zetacizer, and Fourier-transform infrared spectroscopy. Antibacterial effects were evaluated through determining the minimum inhibitory concentration (MIC) of ZnNPs in comparison with chlorhexidine on standard strains of S. mutans and A. viscosus, by microbroth dilution method according to Clinical and Laboratory Standards Institute guidelines. Results: The absorption peak of ZnNPs was in the range of 230–330 nm. The size of ZnNPs varied from 30 to 80 nm, while most nanoparticles were between 50 and 60 nm. The best MIC related to ZnNPs + chlorhexidine was reported with 1.66 and 1.66 μg/ml for S. mutans and A. viscosus, respectively. Furthermore, the lowest MICs related to ZnNPs alone were obtained with 13.33 and 16.33 μg/ml for S. mutans and A. viscosus, respectively. ZnNPs + chlorhexidine in comparison with chlorhexidine had a significant (P < 0.05) antibacterial effect and inhibited the growth of both bacteria. Conclusion: ZnNPs especially combined with chlorhexidine exhibited promising antibacterial effect in comparison with chlorhexidine alone. However, further studies are needed to clarify the accurate mechanisms and toxicity of ZnNPs.