This study was done to clinically evaluate the effect of non–surgical periodontal therapy with or without systemic administration of antibiotic in chronic periodontitis patients. A total of 120 mild to moderate chronic generalised periodontitis patients with probing pocket depth from 4 to 7 mm were randomly divided into three equally sized groups. Patients in Group A treated with scaling and root planing alone; in Group B and Group C the systemic use of amoxicillin (500mg x 3 times daily) and metronidazole (400mg x 3 times daily) were prescribed for 3 and 7 days respectively as an adjunct to scaling and root planing. Pocket depth, clinical attachment level, bleeding on probing and full mouth plaque scores were assessed at baseline, 2 weeks and 6 weeks post treatment. This study represents significantly better clinical outcomes, i.e. Probing pocket depth (PD) reduction and clinical attachment level (CAL) gain, following the adjunctive use of amoxicillin plus metronidazole to scaling and root planing compared to the treatment with scaling and root planing alone. The adjunctive use of amoxicillin plus metronidazole with scaling and root planing for 3 days is enough to gain improvement in clinical outcomes; so that the long–term use of antibiotic that causes unnecessary microbial resistance can be avoided.