Abstract: Objective To study the effects of the different concentrations of tea polyphenols and grapefruit-seed extract mixtures on water- insoluble glucan of Streptococcus mutans (S.mutans), and to further explore the cariostatic mechanism of S.mutans in order to provide much more profitable experimental proof for the clinical practice of the mixture. Methods The 4 mixtures(1/2、1/4、1/8、1/16 MIC) of TP and GSE sub-MIC were cultivated with S.mutans. The supernatant was collected and to measure the content of extracellular polysaccharide with anthrone method. Results The content of water insoluble glucan of S.mutans decreased gradually when the concentrations of TP and GSE mixtures increased. However, in terms of water soluble glucan content, compared with control group, only the experimental group with the highest concentration showed obvious difference (P<0.05). Conclusion The mixture of TP and GSE can effectively inhibit WIG of S.mutans, but it has no obvious inhibitory effect on water-soluble polysaccharide of S.mutans.