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Background and aim: Dental caries is a major health problem nowadays. polyphenolic compounds of tea can exert its anti-cariogenic effects by affecting the cariogenic Streptococcus Mutans bacteria. The aim of this study was to investigate the antimicrobial effect of ethanolic, methanolic and aqueous extracts of green tea in comparison with chlorhexidine on Iranian strain of this bacteria. Materials and Methods: 10 g of green tea powder was dissolved in 100 mg of ethanol, methanol and aqueous solvents separately. After one week, the obtained solutions were filtered and 100mg / ml solution was extracted. to evaluate the diameter of inhibition, zone The Swabs impregnated with suspension scrubbed on culture medium. the extracts were injected onto special paper discs and the impregnated disks were placed on the bacterial culture media, plates were kept in the incubator for 24 hours at 37 °C and then the diameter of inhibition zones was calculated. Using Muller Hinton Broth, herbal extracts were made at concentrations 1:2 (50 mg/ml) and 1:4 (25 mg/ml) and 1:8 (12.5 mg/ml) and 1:16 (6.25 mg/ml) and 1:32 (3.125 mg/ml). Then, 100 μl microbial suspensions made by standard McFarland 0.5 were transferred to all test tube. The test tubes were placed in an incubator at 37°C for 24 hours. After 1 day, contents of all test tubes were separately cultured on Muller Hinton Broth medium. The culture media were incubated at 37°C for 1 day. The results were evaluated in the form of growth or non-growth of bacterial colonies on culture media and MBC and MIC were determined. ANOVA and Post hoc test was used for statistical analyze with SPSS Software. Results: Experimental results after three iterations showed mean values of 16.16, 16.66, 12.5 and 17.5 mm for diameter of the inhibition zone of ethanolic, methanolic and aqueous extracts and chlorhexidine 0.2%, as control. Also, at the significant level of 0.05, there was no significant difference between the diameter of the inhibition zone of ethanol, methanol and chlorhexidine extracts. (p>0.05)The Aqueous Extract Showed Significant difference with other Extracts. MBC Reported 12.5mg/ml for Ethanolic and 50mg/ml for two other extracts. MIC Reported 6.25mg/ml for ethanolic and 25mg/ml for two other extracts. Conclusion: The results showed that the antimicrobial effects of ethanolic and methanolic extract of Iranian green tea are higher than aqueous extract and there is no significant difference between these two extracts with Chlorohexidine. [ABSTRACT FROM AUTHOR]