Objective To evaluate the effect of photo-activated disinfection(PAD)as a kind of adjuvant treatment on moderate to severe chronic periodontitis.Methods 21 patients with the chronic periodontitis(totally 218 selected sites)were randomly enrolled and divided into group A(minocycline hydrochloride),group B(PAD),group C(PAD + minocycline hydrochloride),and group D(no adjunctive therapy)for the adjunctive treatment after receiving the scaling and root planing(SRP).Periodontal indexs as probing depth(PD),bleeding on probing(BOP)and clinical attachment loss(CAL)were examined at the baseline,6 and 12 weeks after the treatment.Meanwhile,periodontal pathogens as Porphyromonas gingivalis(Pg)and Tannerella forsythia(Tf)from subgingival plaque of group A,B and C were detected by Real-time PCR.Results Compared with the baseline,the periodontal inflammations of all groups were improved signiffcantly at 6 and 12 weeks after the treatment(P<0.001),and group A,group B and group C were better than group D(P<0.001),group C was better than group A(P<0.001);Furthermore,the concentration of Pg and Tf was decreased significantly(P<0.001),and there was no difference among the three groups with adjunctive therapy.Conclussion As the adjunctive treatment of SRP,PAD could achieve the same and even better effect than minocycline hydrochloride ointment.