The invention provides an application of extremely giant megamonas in treating or preventing rheumatoid arthritis or related diseases thereof, and also provides extremely giant megamonas containing pharmaceutical compositions, medicaments, foods and feeds. By providing extremely giant megamonas for animals, an anti-inflammatory effect can be effectively played, the symptoms such as redness and swelling of joints are effectively improved, and the symptoms of arthritis are obviously relieved, and therefore, the extremely giant megamonas can be effectively used for treating or preventing rheumatoid arthritis or related diseases thereof.
