Objective: To evaluate the effects of a commercial bleaching agent containing 36% hydrogen peroxide on the clinical plaque index and Streptococcus mutans and Lactobacilli counts in dental plaque and saliva in vivo. Methods: Twenty volunteers were recruited in the present study. Evaluation of plaque index and bacteria counts in plaque and saliva was performed before bleaching the teeth, and 3 days, 2 weeks and 4 weeks post-treatment. Results: Quigley-Hein Plaque Index and Modified Navy Plaque Index were significantly lower on day three after bleaching compared to that before bleaching and the other time points after bleaching. The total number of bacteria in the plaque on day three after bleaching was also less than that before bleaching and the other time points after bleaching. The Streptococcus mutans counts in the plaque and saliva four weeks after bleaching were decreased significantly compared to that before bleaching. Conclusion: Hydrogen peroxide bleaching can decrease plaque accumulation and cariogenic bacteria for a short period of time. Keywords: cariogenic bacteria, peroxide, plaque