The aim of this study was to compare the effect of the topical application of co-amoxiclav gel with placebo in the treatment of adult periodontitis by conducting clinical, microbiological and radiologic measurements. A randomized double-blind placebo-controlled clinical trial design was employed. Subjects were12 patients with adult periodontitis divided into two control and experimental groups both initially received subgingivalscaling and root planning therapy.Clinical, microbiological andradiologic examinations were carried out before treatment and on days 15, 45, and 90 after gel treatment in the experimental group. Radiographic analysis was performed using Trophy radiovisiography (RVG) system determining bone density and bone loss in the study sites. Paired-t test, Wilcoxon matched-pairs test, and binominal test were used for statistical analysis. The statistical analyses showed that both treatments were effective in reducing PPD and BOP over the 3-month period.At the end of follow-up period, the mean reduction in PPD and BOP were1.71 mm and 54.83%, respectively.The increase in mean of alveolar bone crest density (38.16mm) was statistically significant in gel treatment (p<0.03) but it was not significant in placebo group. There was a significant difference between the two treatments with respect to the reduction inproportions of anaerobic gram-negative bacilli during 0-15 and 0-90 day periods. It was concluded that the useof a topically appliedco-amoxciclav 25% gel seems to be effective as aconventionalmechanical therapy in the treatment of adult periodontitis.