Objective To detect the putative periodontal pathogens in the subgingival plaque of chronic periodontitis ( CP) patients with type 2 diabetes mellitus ( DM) and to discuss the role of the periodontal pathogens in the CP and type 2 DM patients. Methods The study included 154 CP patients with type 2 DM ( DM graup), one hundred and twenty subjects with CP (CP group) and 40 healthy subjects. Subgingival plaque samples were obtained with sterile subgingival curettes and pathogens genomic DNA were extracted by phenol and chloroform from plaque. Taking Porphyromonas gingivalis ( Pg) , Actinobacillus actinomycetemcomitans ( Aa ) , Fusobacterium nucleatum ( Fn) , Prevotella intermedia ( Pi) , Tannerella forsythia(Tf) and Treponema denticola(Td) as aim pathogens, polymerase chain reaction (PCR) method was used to determine the prevalence of Pg, Aa,Fn,Pi,Tf and Td in the subgingival plaque samples.Results Pg, Aa, Fn, Pi, Tf and Td were all found in the subgingival plaque of DM group. In serious periodontitis, the detection rate of Pg, Aa and Tf in DM group [ Pg:78% (72/92) , Aa:27% (25/92) ,Tf: 67% (62/92) ] was significantly higher than in the CP group[ Pg:58% (35/60) , Aa: 17% ( 10/60) ,Tf: 43% (26/60) ] ,P <0. 05. In slight periodontitis, the detection rate of Pi in DM group was significantly lower than in the CP group. At the same time, the average gradation(AVG) ratio of PCR product of Aa and Tf in DM group was significantly higher than in the CP group and the AVG ratio of Pi in DM group was significantly lower than in the CP group ( P < 0. 05 ). Conclusions Compared with the pure CP, the quantity of Pg, Aa and Tf in the subgingival plaque of the periodontitis patients with type 2 DM increased, but the Pi quantity reduced.