The purpose of this study was to evaluate the inhibitory effects of herb extracts on caries-related bacteria and glucan of dental plaque in vitro. Bacterial sensitivity tests were done using broth dilution, and the phenol sulphate method was used to assess glucan inhibition. The results showed that tannic acid could inhibit bacterial growth more effectively than other herb extracts. Eugenol showed a 46.87 +/- 12.74 and 36.67 +/- 6.30% inhibitory effect on insoluble and soluble glucan synthesis, respectively. Cnidium, barbaloin, caryophyllin, and piperine had > 40.0% inhibitory effect on soluble glucan synthesis. Both insoluble and soluble glucan synthesis of S. sobrinus were inhibited by eugenol and piperine. Eugenol and piperine were efficacious in inhibiting glucan synthesis making them desirable agents for oral care products.