Abstract： Objective To examine Porphyromonas gingivalis (Pg) in subgingival plaque of the patients with periodontitis and to find out the rules of Pg colonization after periodontal initial treatment. Methods A total of 1620 subgingival plaque samples were collected from 180 subjects including chronic periodontitis (CP) patients (n = 90), and aggressive periodontitis (AgP) patients (n = 90) in different periods of periodontal initial therapy -- the baseline, 6 weeks, and 12 weeks after treatment. The following periodontal clinical parameters were recorded with Florida probe at sampled sites : probing depth ( PD) ,clinical attachment loss ( CAL), and bleeding on probing ( BOP). Quantities of Pg were examined by AmpliFluor endpoint quantitative polymerase chain reaction. Results At the 6th week of periodontal initial therapy, there were 61 (22. 6% ) and 66 (24.4%) Pg increased sites respectively, in which no significant difference was detected ( P > 0. 05 ). At baseline of periodontal initial therapy, more severe periodontal clinical parameters of Pg increased sites were observed than those of Pg stationary sites. At the 12th week,however, there were 96 (35. 6% ) and 18 (6. 7% ) Pg increased sites respectively, significant difference detected (P < 0. 05). At 6th week of periodontal initial therapy, more severe periodontal clinical parameters of Pg increased sites were observed than those of Pg stationary sites. Conclusions Pg colonization in AgP and CP patients started 6 weeks after periodontal initial therapy, but the recolonization pattern was differentbetween these two groups of patients. Severe periodontitis sites in baseline seemed to place them at risk of Pg colonization after periodontal initial therapy.