Introduction & Objective: Dental caries is an infectious disease caused by colonization of Streptococcus mutans bacteria, which is begun by decalcification of non-organic part of teeth and continued with destruction of organic matrix. According to the incidence of relative resistance of biocide and also antibiotics in the normal micro flora of mouth specially viridians species and recent trends in the use of medicinal herbs to prevent and treat diseases due to fewer side effects, in this study, the MBC and MIC effect of hydro alcoholic extract of Mentha  Piperita and NaCl on S. mutans in vitro were evaluated.

Materials & Methods: In this experimental study hydro alcoholic extract has been prepared from Mentha Piperita with Maceration method (soaking in a solvent). After the preparation of the S. mutans strain standard and sterilization of hydro alcoholic extract by filtering, the antibacterial effects have been evaluated by Agar well diffusion, Agar disk diffusion and broth micro dilution methods (Concentration in the range of  0.78 - 800 micrograms per milliliter (µg/ml)). Meanwhile, the concentration range of sodium chloride 0.5 - 10 % was prepared and its effect on the test bacteria was determined.

Result: In Agar well diffusion method and Agar disk diffusion method the hydro alcoholic extract of Mentha Piperita and NaCl had no antibacterial effect on S. mutans. In broth micro dilution method, hydro alcoholic extract of Mentha Piperita had no antibacterial effect on S. mutans but the MIC and MBC of NaCl was 5%  and  5.5% , respectively.

Conclusion: Our results indicated that the hydro alcoholic extract of Mentha Piperita had not any antibacterial property against S. mutans but the most effective antibacterial concentration of NaCl was 5 - 5.5 percent.