Enamel decalcification usually can be found around orthodontic bracket after debonding. Orthodontic adhesives resistant to bacterial colonization were used to prevent white spot lesion and dental caries in orthodontic treatment with fixed appliance. The objectives of this study were to evaluate the antibacterial properties of expeimental orthodontic adhesive containing gold nanoparticles (AuNPs). 108 orthodontic adhesive discs containing 1.0 wt%, 0.5 wt%, 0.25 wt% AuNPs and without AuNPs were prepared from in-house orthodontic adhesive (27 discs per group). The antibcterial properties of adhesive discs were evaluated by direct contact test. Streptococcus mutans and Streptococcus sobrinus suspensions were placed on the discs and incubated at 37 °C for 1 hour. The adhesive discs were transferred to BHI broth and were incubated at 37°C for 16, 20 and 24 h. Bacterial growth was evaluated by optical density (OD) measurement at 590nm. The results showed that experimental orthodontic adhesives with 1.0 wt% and 0.5 wt% AuNPs exhibited higher antibacterial activity compared to the control group (p < 0.05). At 24 h of incubation, the median OD of 1.0 wt%, 0.5 wt%, 0.25 wt% and control in S. mutans were 0.109, 0.006, 0.007 and 0.372, rescpectively. In S. sobrinus, the median OD were 0.173, 0.012 , 0.007 and 0.328. The incorporation of gold nanoparticles into orthodontic adhesives exhibited antibacterial activity against cariogenic microorganism. At a concentration of 0.5 wt% and 1.0 wt% AuNPs, experimental orthodontic adhesive could reduce bacterial growth of both S. mutans and S. sobrinus.