Objectives: The aim of this study was to examine the antibacterial effect of mastic oil on a representative caries-inducing bacterium, Streptococcus mutans. Methods: S. mutans UA159 was maintained in a BHI medium containing 0.1% sugar at 37℃ and 10% CO2 . Strains were treated with six different concentrations of mastic oil (0, 0.1, 0.2, 0.3, 0.4, 0.5%). After incubation for 16 h, the antibacterial effect was evaluated by measuring the optical density (OD) and observing the colony-forming unit. A microtiter plate assay was performed to analyze the anti-adhesion ability of mastic oil on S. mutans. Results: More than 0.1% of mastic oil inhibited the growth of S. mutans. In addition, 0.4% mastic oil exterminated S. mutans. Mastic oil induced an anti-adhesive ability in S. mutans. Conclusions These results suggest that mastic oil may be used as a preventive measure against dental caries.