Streptococcus mutans (SM) is one of the most important bacterial pathogen for dental caries. Glucosyltransferases (GTF) are a group of enzymes play as key factors for its virulence. The combination of extranase (Dex) and sodium fluoride (NaF) is reported could reduce the biofilm of SM. This study is to investigate and compare the different inhibitory effects of exogenous Dex alone or with NaF on the activity of glucosyltransferase in solution or adsorbed to a surface. GTF produced by Streptococcus mutants ATCC 25175 was purified and the purity and concentration were determined with SDS-PAGE and BCA protein quantitation respectively. GTF was either dissolved or adsorbed to the surface of saliva-coated hydroxyapatite beads and incubated with ¹⁴C-labelled sucrose. Exogenous Dex and NaF at various combined concentrations was added. The quantity of sucrose transformed per minute was determined with a scintillation counting technique. Exogenous Dex inhibited GTF activity both in solution and adsorbed state (p < 0.05). The inhibitory effect on GTF in solution was stronger (65%) than in adsorption (45%). The addition of NaF did not increase the inhibitory action of Dex (p > 0.05). The direct inhibition of GTF activity by Dex is a possible mechanism underlying the management of SM biofilms with exogenous Dex.