The antibacterial activity of Sinapis alba seed oils was tested against oral bacteria. The active constituent of S. alba oil was identified as phenethyl isothiocyanate. Phenethyl isothiocyanate showed strong and moderate inhibition at 0.75 and 0.5 mg/disc, respectively, against Porphyromonas asaccharolytica, P. gingivalis and Streptococcus sobrinus. Similarly, benzoyl isothiocyanate and benzyl isothiocyanate had moderate inhibition at 0.5 mg/disc against P. asaccharolytica, P. gingivalis and S. sobrinus. These results indicate that the effectiveness of isothiocyanate derivatives against oral pathogens is related to their aromatic structure.