Statement of Problems: Incomplete therapy of periodontal diseases treated with scalling and root planning (SRP) has been resulted in improvement of host- modulated therapy (HMT) as a new method.Purpuse: Introducing an adjunctive therapy in addition to SRP in managing of destructive preiodontal therapy was the main purpose of this study.Materials and Method: Fifteen patients suffered from chronic periodontitis were randomly assigned to test and control groups. Treatment consisted of: SRP+ placebo (control group), SRP+ low dose doxycycline (LDD) twice daily for six months (continous LDD), and SRP+LDD twice daily for first and third two months after completion of phase I (cyclic LDD). Gingival indices including: probing depth (PD), clinical attachment level (CAL), modified gingival index (MGI), and O’leary plaque index were measured and recorded at baseline and at one and six months after the completion of therapy. The data were analyzed using one-way ANOVA and Kruskal-Wallis tests.Results: Probing depth reduction in both of LDD groups were significantly greater than control group (p<0.01). However, this reduction happened between baseline and follow-up oppointments. In other word, the differences between the 1-and 6-month evaluation were not significant (p=0.25). Continous LDD regimen showed an additional PD reduction (from 5.40 to 2.06mm) for pockets greater than 5mm in comparison to the other groups.Conclusion: Within the limitation of this study, the results showed that adjunctive continous and cyclic regimen of low-dose doxycycline can improve the clinical efficacy of conventional therapy, even in patients with less than favorable plaque control.Key words: Chronic periodontitis, Antibiotic, Doxycycline, Adjunctive therapy.