OBJECTIVE：To study the in vitro anti-bacterial activity and potential mechanism of ellagic acid on Streptococcus mutans，and to provide evidence for its prevention and treatment of dental caries. METHODS ：Using Compound chlorhexidine gargle as positive control ，5％DMSO as negative control ，bacteriostasis experiment was conducted by the method of drilling hole ， and bacteriostatic effects of 50，25，12.5，6.25，3.125，1.562 5 mg/mL ellagic acid on S. mutans was preliminarily determined by measuring the diameter of bacteriostatic ring. The minimal inhibitory concentration （MIC）and minimal bactericidal concentration （MBC）of ellagic acid on S. mutans were determined by microdilution method. Using 5％ DMSO as negative control ，the effects of 1/8 MIC，1/4 MIC，1/2 MIC and MIC ellagic acid on the formation of S. mutans biomembrane was determined by crystal violet staining. The changes of the biomembrane structure under the action of 1/2 MIC ellagic acid were observed by microscopy after fluorescence staining. Phenol sulfuric acid method and reducing coenzyme Ⅰoxidation method were used to determine inhibitory effects of 1/8MIC，1/4MIC，1/2MIC，MIC ellagic acid on S. mutans on extracellular polysaccharide （EPS）as well as effect on the activity of lactate dehydrogenase （LDH）in extracellular matrix. RESULTS ：Ellagic acid with concentration of 12.5～50 mg/mL produced an inhibitory ring on S. mutans with diameter greater than 15 mm. Under the action of 50 mg/mL ellagic acid ，the diameter of bacteriostatic ring was the same as that of Compound chlorhexidine gargle . MIC and MBC of ellagic acid to S. mutans were 12.5 mg/mL and 25 mg/mL. The survival rate of bacterial biomembrane after 1/8MIC-MIC ellagic acid treatment was significantly lower than that of the negative control （P＜0.01），and had a certain dose-response trend. After MIC ellagic acid treatment，the survival rate of bacterial biomembrane was （16.41±1.346）％. After fluorescence staining ，the structure of bacterial biomembrane was destroyed by 1/2 MIC ellagic acid. After treated with 1/8MIC-MIC ellagic acid ，its inhibitory rates on water-soluble EPS and water-insoluble EPS were increased significantly ，compared with negative control （P＜0.01）. After treated with 1/4MIC-MIC ellagic acid ，the activity of LDH in the extracellular matrix of bacteria increased significantly ，compared with negative control （P＜0.01），in dose-effect dependent trend. CONCLUSIONS ：Ellagic acid can inhibit the growth of S. mutans ，the mechanism of which may be associated with inhibiting EPS production ，reducing bacterial adhesion ，destroying bacterial cell membrane.