The objective of this study is to investigate the inhibitory effect of the crude extracts from some herbs on adherence of Streptococcus mutans (S.mutans) ATCC 25175 and TPF-1 in vitro. Six herbs ie. Streblus asper, Chinese black tea (Camellia sinensis); Cassia alata; Andrographis paniculata; guava (Psidium guajava) and Harrisonia perforata were extracted with 50% or 95% ethanol and dried. Herbal extracted solution at 0.5% concentration (w/v) was initially tested for bacterial adherence on glass surfaces. The extracts that showed the inhibition on glass surfaces were then tested on saliva-coated hydroxyapatite by use of radiolabeled bacteria, in order to identify type and effective concentration of the extracts. To study the mechanism of action, the effect of the extracts at such concentration on glucosyltransferase and glucan-binding lectin activities were examined. It was found that all extracts, but Streblus asper, showed significant inhibitory effect on bacterial adherence to glass surfaces. For saliva-coated hydroxyapatite adherence assay, Chinese black tea, Cassia alata, Andrographis paniculata and Harrisonia perforata could inhibit adherence of S.mutans ATCC 25175. Chinese black tea was the strongest inhibitor followed by Andrographis paniculata, Cassia alata and Harrisonia perforata, respectively. For S.mutans TPF-1, adherence inhibition was observed from Cassia alata and Andrographis paniculata at similar level. The lowest concentrations of the extracts that inhibited the adherence at least 50% were 0.3% of Chinese black tea, 0.5% of Cassia alata, 0.5% fo Andrographis paniculata and 0.5% of Harrisonia perforata for S.mutans ATCC 25175. For S.mutans TPF-1, the effective concentrations were 0.4% of Cassia alata and 0.5% of Andrographis paniculata. All extracts at such concentrations decreased the activity of glucosyltransferase from both strains. Only Cassia alata and Andrographis paniculata inhibited or decreased the activity of glucan-binding lectin from both strains. These findings suggested that Chinese black tea, Cassia alata, Andrographis paniculata and Harrisonia perforata could inhibit adherence of S.mutans ATCC 25175, when Cassia alata and Andrographis paniculata had effect on S.mutans TPF-1 in vitro at the concentrations employed in this study.