The purpose of this study was to investigate the effect of cranberry polyphenol fraction on mutans streptococci. Hydrophobicity is an important factor in the adherence of bacteria to the tooth surface. We found that cranberry polyphenol fraction significantly decreased the hydrophobicity of Streptococcus sobrinus 6715, Streptococcus mutans MT8148R and JC2 in a dose-dependent manner (p<0.05). Biofilm formation by S. sobrinus 6715 and S. mutans MT8148R was inhibited by 100 μg/ml cranberry polyphenol fraction (p<0.01). When dosage was increased to 500 μg/ml, biofilm formation by S. mutans JC2 was significantly inhibited (p<0.05). Addition of 500 μg/ml cranberry polyphenol fraction to medium inhibited growth of S. mutans MT8148R compared with the control (p<0.05).