It has been reported that dried orange peel in a medicine prepared from crude drugs has antioxidant, anti-inflammatory, anti-allergic, anti-bacterial, and anti-viral functions, prevents circulatory diseases, reduces lipids, improves immunity, and strengthens capillary vessels. This study attempted to identify the antibacterial effects of dried orange peel extract on S. mutans causing dental caries, and its effects on GTase activation. The extract was put on the medium at the concentration of 5, 10 and 20 mg/ml and its effects on the inhibition of S. mutans growth were examined. Analysis found that the higher the concentration was, the more the number of colonies was reduced. In accordance with the measurement after 8 hours, the extract displayed a high growth inhibition rate: 92% at a concentration of 5 mg/ml, 95% at a concentration of 10 mg/ml and 99% at a concentration of 20 mg/ml. Furthermore, the GTase deactivation rate by the extract was measured. The higher the concentration of extract was, the more rapidly the deactivation rate increased. The deactivation rate of the extract was significant at 42% at a concentration of 5mg/ml.