Streptococcus mutans, a typical dental caries-causing oral pathogen, forms dental biofilm by attaching to the surface of teeth. Its glucosyltransferase (GTFase) is responsible for synthesizing water-insoluble glucan from sucrose, which is an important factor for biofilm formation and for providing microbial resistance against stresses. Therefore, inhibiting the activity of GTFase is an effective approach to prevent the formation of dental biofilm even without killingS. mutans. In this study, we found that 2.5 mg/mL of methanol extracts ofCamellia sinensisleaf,Diospyros kakileaf,Nelumbo nuciferaseed,Rubus coreanusfruit, andUlmus davidianarhizodermis inhibits both GTFase activity and biofilm formation ofS. mutanswithout affecting cell growth. Phenolic acids such as ferulic acid, salicylic acid, and vanillic acid inR. coreanusfruit extract inhibited GTFase activity. Enzyme kinetic analysis shows that ferulic acid, salicylic acid, and vanillic acid are competitive, noncompetitive, and uncompetitive inhibitors, respectively. These results suggest thatR. coreanusfruit extracts containing phenolic acids may control and prevent dental caries without killingS. mutansby inhibiting GTFase activity.