Stevia rebaudiana is grown commercially in Thailand. The extracts of Stevia rebaudiana leaves have been accepted for use as sweetener in many countries. Objective: The objective of this study was to determine the effects of Stevia extract on the caries-inducing properties of Streptococcus sobrinus in vitro. Methods: Growth rate, acid production, extracellular polysaccharides production, cell surface hydrophobicity, cellular aggregation and adhesion properties of S. sobrinus 6715 were studied in the presence and absence of stevia extracts using the modified methods described by Matsumoto et al. The effects of xylitol on these properties were also performed. Results: Stevia extract at concentrations of up to 10% had no effect on the growth rate of streptococci and acid production, while 10% xylitol showed 50% inhibition. On the other hand, the extract decreased cell surface hydrophobicity, induced cellular aggregation, inhibited extracellular polysaccharides production and inhibited the adherence of bacteria to saliva coated plates. Conclusions: These results suggested that although stevia extract at concentrations of up to 10% had no effect on acid production and growth rate of streptococci, it may inhibit bacterial adherence by reducing hydrophobicity and inhibition of extracellular polysaccharides synthesis.