Abstract: OBJECTIVE To explore the influence of fixed appliance on Porphyromonas gingivalis in children receiving orthodontic treatment. METHODS A total of 80 children who received the orthodontic treatment by using fixed appliance in Affiliated Hospital of Hangzhou Normal University from Jan 2013 to Dec 2015 were recruited as the study objects.The periodontal indexes such as probing depth, gingival sulcus bleeding index, dental plaque index, loss of attachments and change of P.gingivalis were observed before the orthodontic treatment and after the treatment for 1 month, 3 months and 6 months. RESULTS The probing depth, gingival sulcus bleeding index and dental plaque index were higher after the treatment for 1 month than before the treatment (P＜0.05); the gingival sulcus bleeding index and dental plaque index were significantly higher after the treatment for 3 months than before the treatment (P＜0.05); there was no significant difference in the probing depth, gingival sulcus bleeding index or dental plaque index before the treatment and after the treatment for 6 months.All of the 80 children were not detected with the loss of attachments before and after the treatment.The isolation rate of P.gingivalis was 15.0% before the treatment, 31.3% after the treatment for 1 month, 35.0% after the treatment for 3 months, 17.5% after the treatment for 6 months; the isolation rate of P.gingivalis was higher after the treatment for 1 moth and 3 months than before the treatment (P＜0.05), there was no significant difference in the isolation rate of P.gingivalis after the treatment for 6 months and before the treatment.The percentage of P.gingivalis was (0.082±0.034) after the treatment for 1 month, (0.097±0.031) after the treatment for 3 months, significantly higher than (0.004±0.001) before the treatment(P＜0.05); there was no significant difference in the percentage of P.gingivalis after the treatment for 6 months and before the treatment. CONCLUSIONThe orthodontic treatment with the fixed appliance may lead to the increase of periodontal probing depth, gingival sulcus bleeding index and dental plaque index as well as the rise of isolation rate of P.gingivalis; the periodontal indexes may basically returned to normal after the orthodontic treatment for 6 months.