Sixty smoker participants with a history of moderate to severe chronic periodontitis were selected from the population referred to the Department of Periodontics, international branch of Shiraz University of Medical Sciences. A detailed medical, periodontal, dental, and smoking history was obtained. Individuals who fulfilled the inclusion/exclusion criteria were invited to participate in the study. All eligible participants were thoroughly informed of the nature, potential risks, and benefits of their participation in the study and provided written informed consent. They were also informed about the known side effects of tinidazole and possible gastrointestinal disturbances. This study protocol was approved by the institutional ethical committee. The participants were randomly allocated to one of two groups .The control group consisted of 30 participants, and the test group had 30 participants. In both groups ages be matched with each other. Before scaling and root planning , Strict oral hygiene instructions (OHI) were instituted on all participants at the same time by the operator .then GI Leo and Silness ، PI Oleary ، BI Lenox ، Attachment level ، Recession ، PD measured by use of Williams probe. Recession and PD recorded in all present teeth in six sites (mesio-buccal, midbuccal, disto-buccal, mesio-palatal/lingual, mid-palatal/ lingual, and disto-palatal/lingual) in each patient of both groups by the same person. Disclosing tablets was used for evaluating PI in four surfaces of teeth (mesial, distal, buccal,lingual/palatal).BI lenox was measured in four sites (mesial, distal, mid buccal and mid palatal/lingual) in all present teeth. GI index evaluated in six teeth (mandibular first molar, first premolar, central incisors and maxillary first molar, first premolar and central incisors). Next, the operator treated the periodontal disease with SRP. After this, each participant received a package containing the test or placebo medication; all packages were identical in appearance and were marked only with the participant number. Participants in the test group received 2g tinidazole as loading dose in the first day and then participants in the test group received 500 mg tinidazole to be taken once daily for 6 days. Participants in the control group received similar looking placebos prepared at the College of Pharmacy, Shiraz. The treatment group was masked from the patient, clinical examiner, operator, and statistician. Clinical Monitoring: all the participants in both groups were recalled at 6 weeks after the baseline visit. Clinical parameters, including PI, GI, PD, Recession ,BI and CAL, were recorded at these time intervals. OHI were checked.