Objectives: The main factors causing tooth decay are the increase in the S.mutans and Lactobacillus acidophilus. Studying the effects of plants on teeth caries due to the increasing interests on using herbal medicines, their accessibility, low cost, and effectiveness, has led the researchers to one of the most effective ways of preventing this infectious disease. In this in vitro study, the antibacterial effect of black pepper (Piper nigrum L.) and garlic (Allium sativum L.) extracts on standard Streptococcus mutans and Lactobacillus acidophilus were reviewed. Methods: This experimental study was conducted in two stages. At the first stage, alcoholic extracts of black pepper and garlic were prepared by Percolation extraction method and the concentrations of 256 and 512 mg⁄ml of them were diluted by Serial dilution method. Then, at the second stage, the evaluation of minimum inhibitory concentration of the extracts on Streptococcus mutans and Lactobacillus acidophilus was performed by Broth macrodilution method. Furthermore, in order to test the susceptibility of these microbes to extracts, Zone of Inhibition of these extracts was evaluated by Agar well diffusion method. Results: In all repetitions of the test, for the concentrations of 256 and 512 mg⁄ml of extracts, in all tubes, a few turbidity related to the presence of microbes was seen, so, no measures for MIC were determined. Also in Agar well diffusion method, for the concentrations of 256 mg⁄ml of both extracts, no Zone of Inhibition was observed. However, for the concentration of 600 mg⁄ml of black pepper, for both of microbes, a very small Zone of Inhibition, about 1 mm, was observed around the wells. Conclusions: Plant extracts of black pepper and garlic have no significant effect on oral standard Streptococcus mutans and Lactobacillus acidophilus. It is worth mentioning that many factors can affect the outcome of this research.