Dental caries is a major problem of dental health in Indonesia. Streptococcus mutans is a bacteria that can cause caries. In this recent time, the agents used to eliminate the caries is less effective due to S. mutans resistance to those agents. Moreover, the agents may produce side effects after 10 years of consumption.Therefore, effort to find alternative agents against dental caries is needed. Red betel (Piper crocatum Ruiz & Pav) is a promising natural material to treat dental caries. The P. crocatum Ruiz & Pav leaves contain secondary metabolites such as essential oil, flavonoid, alkaloid, and phenolic compounds that active against S. mutans involved in caries formation. This study aimed to evaluate the activity of secondary metabolites isolated from red Betel leaves against S. mutans and its inhibition effect on the glucosyltransferase (GTF) activity. The growth inhibition test of S. mutans was carried out with liquid dilution test and diffusion test. Cultures of S. mutans were treated with essential oil, alkaloid, flavonoid, phenolic compounds isolated from P. crocatum Ruiz & Pav in various concentration and incubated at 37C in anaerob condition. Povidone iodine was used as positive control and aquadest as negative control. The inhibition test of GTF activity was performed by measuring the levels of formed fructose that was analyzed by Somogyi-nelson method. The data was analyzed using one-way ANOVA with a 95% confidence interval.The results showed that essential oil and alkaloid had lowest MIC (Minimal Inhibitory Concentration) compared with the other active compounds (5000 μg/mL) but higher than povione iodine as positive control (2500 μg/mL). The essential oil exhibited similar inhibition zone diameter compared with povidone iodine, either at low or high concentration (p>0.05). The essential oil had similar inhibition activity to the glucosyltransferase compared with povidone iodine (p>0.05). In conclusion, the essential oil of red betel leaves had the strongest activity against the S. mutans growth through its inhibition effect on GTF activity.