Objectives: The aim of the present study is to assess the clinical and microbiological effects of locally delivered moxifloxacin gel at different concentrations(0.5% and 1%) as an adjunct to SRPcompared with SRP alone in the treatment of chronic periodontitis. Patients and Methods: A total of forty patients with moderate chronic periodontitis were selected for this study. The selected patients were divided in two groups (group I and group II) and every group was divided according to the split-mouth design into applied and control side. All patients were subjected to conventional periodontal therapy (SRP), moreover; the applied side of each group was subjected to sub gingival delivery of moxifloxacin gel, the patients followed up for six weeks. Clinical parameters used in this study included; plaque index (PI), gingival index (GI), papillary bleeding index (PBI), probing pocket depth (PPD) and clinical attachment level (CAL). These parameters were recorded for all selected patients to provide clinical evaluation of periodontal status at the beginning of the study (baseline), and at the end of the study after six weeks. In addition; sub gingival plaque sample were taken before starting the treatment and also at the end of the study after six weeks using sterile curette and delivered in a sterile tube containing a suitable transport media and analyzed for isolation of p.intermedia and p.gingivalis. Results: The clinical results of both groups showed statistically significant difference for both applied and control side which were higher in applied side than control one. Regarding to the inter-group comparison; there was no statistically significant difference between both concentrations for all clinical parameters except papillary bleeding index was more effective in low concentration of moxifloxacin gel (0.5%) treated side. Microbiologically, results of microbial evaluation showed that, p.intermedia and p.gingivalis count decreased significantly after treatment in both concentrations especially with high concentration (1%) moxifloxacin gel treated side. Regarding to the inter-group comparison there was no statistically significant difference between two concentrations of moxifloxacin gel. Conclusions: Scaling and root planning (SRP) was effective in improving clinical and microbiological parameters in patients suffering from moderate chronic periodontitis. Local use of moxifloxacin gel as an adjunct to SRP has resulted in significant additional improvement in the clinical conditions of moderate chronic periodontitis patients when compared with SRP alone. There was no additive benefit by increasing concentration of moxifloxacin gel in treatment of moderate chronic periodontitis as adjunctive therapy with SRP.Therefore; using moxifloxacin gel with low concentration (0.5%) would be effective, costless and less side effects.