Dental caries is one of the most common oral diseases in the world. Glucosyltransferase (Gtase) of Streptococcus mutans (S. mutans) plays an important role in the develo pment of dental caries. For the purpose to develop anti- caries, we examined the effect of metabolites isolated from Streptomyces sp. M-20 on Gtase and the growth of S. mutans. Streptomyces sp. M-20 isolated from Mongolian soil showed 95~96% sequence homology with that of Streptomyces lin- colnensis. The metabolites of Streptomyces sp. M-20 were partially purified by extraction with ethyl acetate, silica gel column chromatography and preparative TLC. Partially purified metabolite, red colored component (MR-20) in ethyl acetate fraction showed potent antibacterial activitiy against S. mutans and inhibitory activity against Gtase purified from S. mutans, while another isolated yellow component (MY-20) showed no activity against S. mutans. The inhibitory activity of MR-20 against Gtase was confirmed by activity staining on SDS-polyacrylamide gel electrophoresis. The concentration of MR-20 for 50% inhibition $(IC_{50})$ against Gtase activity was $60{mu}g/ml$. These results suggest that MR-20 can be developed for antibacterial agent and anticaries.