Microbial infections are believed to play an important role in the initiation and perpetuation of rheumatoid arthritis. This study aimed to investigate the relationship between the presence of Porphyromonas gingivalis DNA in the synovial fluid and rheumatoid arthritis. The synovial fluid samples were collected from 22 patients with rheumatoid arthritis and 20 patients with not suffering from rheumatism, overall 42 patients were investigated. The presence of P. gingivalis DNA was evaluated by the real-time PCR method. There was a significant relationship between rheumatoid arthritis and non-rheumatoid arthritis with the DNA number (Pv< 0.05). P. gingivalis DNA were detected in 3 of 20 (13.6%) rheumatoid arthritis patients, but there was no significant relationship between P. gingivalis DNA and rheumatoid arthritis (Pv> 0.05). DNA of periodontal pathogens can be found in the synovial fluid of rheumatoid arthritis patients. It shows oral bacteria may play a role in the pathogenesis of rheumatoid arthritis.