The invention provides application of actinomyces naeslundii in treating or preventing rheumatoid arthritis or related diseases thereof, and further provides a pharmaceutical composition, a medicine, a food and feed containing the actinomyces naeslundii. Applied to animals, the actinomyces naeslundii can be used for effectively developing an anti-inflammatory efficacy, so that the symptom of redness and swelling of joint is effectively improved and the symptom of arthritis is remarkably relieved and the actinomyces naeslundii can be effectively used for treating or preventing rheumatoid arthritis or related diseases thereof.
