This study was executed to evaluate the effects of natural herbal medicine extract on the antimicrobial activity and the artificial plaque formation. Schisandra chinensis, Sophora flavescens, Dryopteris crassirhizoma and Aristolochia contorta were extracted by 80% MeOH respectively and each concentration(5mg, 10mg, 20mg) of herbal medicine extract was tested to show the antimicrobial activity against S. mutans. To show the inhibition effect of herbal medicine extract on the artificial dental plaque formation each extract of 0.4% concentration was added into M 17 broth and inoculated with $5.0{ imes}10^6$ of S. mutans. After 24 hour incubation each weight of artificial dental plaque was measured and compared with each other. Results obtained are as follows: 1. Dryopteris crassirhizoma showed very intense antimicrobial effect but Schisandra chinensis, Sophora flavescens and Aristolochia contorta showed a little difference against control. 2. Regarding artificial plaque formation inhibiting activity by each herbal extract at 0.4% concentration, Dryopteris crassirhizoma showed the perfect inhibition effect. Schisandra chinensis and Aristolochia contorta followed in order but Sophora flavescens showed the worst value. In conclusion, Dryopteris crassirhizoma showed strong antimicrobial effect against S. mutans and significant inhibiting effect on the artificial dental plaque formation.