Background: To evaluate the antibacterial property of a modified orthodontic resin with different concentrations of silver-nanoparticles (AgNPs), and quantify its preventive effect on the formation of white spot lesions (WSLs). Material and Methods: An orthodontic resin (Transbond XT) was modified with four concentrations of AgNPs (1%, 0.5%, 0.1%, and 0.05%), the orthodontic resin without AgNPs was used as control. Polymerized resin discs (n=80) were submitted to Agar diffusion test on Petri dishes inoculated with Streptococcus mutans and Lactobacillus acidophilus. In addition, resin discs of each group (n=40) were placed in 96-well plates with bacterial suspensions to evaluate the colony-forming-units (CFU). For the WSLs prevention test, brackets were bonded with the experimental orthodontic resins on 45 premolars (n=5), and were subjected to a microbiological caries induction method for 9 days. Photographs were taken before and after the test, and the images were evaluated with the Image J software to calculate the area of WSLs. The data were analyzed using ANOVA and Tukey-HSD test, Student's t-test and Kruskal-Wallis test (a=0.05). Results: The 0.5% and 1% AgNPs modified resin inhibit the growth of S. mutans and L. acidophilus. All the modified resins showed significantly less CFU, when compared to the control (p<0.05). 1% AgNPs resin promote the higher prevention of WSLs formation. There was no significant difference between the control group and the 0.1% and 0.05% groups. Conclusion: 0.5% and 1% of AgNPs modified orthodontic resin exhibit an important antibacterial activity against S. mutans and L. acidophilus, and prevent the formation of WSLs.