Background: Bacteria play an essential role in the process of dental caries. Caries prevention efforts are made in many ways using an antiseptic or herbal mouthwash including avocado leaves that have the effect of antifungal, antimicrobial, analgesic, and anti-inflammatory.The Objective: This study aims to determine the effectiveness of avocado leaf extract concentration of 60% and 80% for blocking the inhibition of the Streptococcus viridans, the bacteria that has a crucial factor in the formation of dental caries.Method: The research was a quasi-experimental research using two samples in each treatment group. The sample consisted of 3 treatment groups of young avocado leaf extract concentration of 60%, 80%. Methods of data analysis used descriptive quantitative research.Result: The results showed that the leaf extract of avocado with a concentration of 60% does not affect inhibiting the growth of bacteria Streptococcus viridans with an average of inhibition of 26.43mm. However, the avocado leaves extract with a concentration of 80% affect the growth of Streptococcus viridans bacteria with an average inhibition of 27.6 mm.Conclusion: The conclusion of this study proves that avocado leaf extract has potent antibacterial activity against the growth of Streptococcus viridans.