The antimicrobial activity of Sappan Lignum against S. mutans has been reported by You et al. 19 However, we widely confirmed the antimicrobial activity of Sappan Lignum about periodontal diseases as well as dental caries. In our study, the aqueous extract of Sappan Lignum showed strong antimicrobial activity against all five species of oral bacteria. The MIC values of Sappan Lignum against E. faecalis and A. viscosus were especially low relative to those of the other herbal medicines. Further, Sappan Lignum extract had strong activity against oral bacteria in the agar diffusion assay; this activity was more pronounced for Streptococcus spp. than for other species such as E. faecalis and A. viscosus. Therefore, we have provided primary data showing that Sappan Lignum extract could be a potential treatment for both dental caries and periodontal diseases.