In order to find out an anticariogenic substance, antibacterial activities of some medical herbs and spices against Streptococcus mutans were investigated. The essential oils of oregano, thyme, sage, fennel, nutmeg, rosemary, calamus and cassia cortex were shown antibacterial effect against S. mutans. The essential oil of oregano among other tested medical herbs and spices showed the most active antimicrobial activity. The essential oil of oregano inhibited the activity of glucosyltransferase from S. mutans. The minimal inhibition concentration of oregano essential oil was determined as 0.05nl/ml. The activity of glucosyltransferase from S. mutans was inhibited in the presence of oregano essential oil.