For evaluating the effect of Siegesbeckiae Herba (here after abbreviated as SBH) on rheumatoid arthritis, the experiment was carried out; Incidence of rheumatoid arthritis, IRA(indices of rheumatoid arthritis), immunophenotypes by flow cytometer and histopathological changes in MRL/MpJ-Ipr-Ipr Mice in vivo were studied. The results were obtained as follows: 1. Incidence of RA in MRL/Ipr mice was suppressed to 60% of control by SBH. 2. IRA was significantly reduced for IgG3 and IgM at 20 weeks of age and for IgG2b at 12 and 20 weeks of age in mice by SBH compared with control. 3. Immunophenotypes such as CD4⁺/CD25⁺, CD8⁺/CD3e⁺, CD69⁺/B220⁺, NK⁺/CD3e⁺ were significantly increased by SBH compared with control. 4. In histopathological analysis, SBH suppressed the progression of PMN(polymorphonuciear leukocyte), leukocyte and fibroblast infiltration, and subsynovial soft tissue edema frequently showing in the early stage of the arthritis, and also effectively reduced the degeneration of cartilages and degenerative bone symptoms. These results suggest Siegesbeckiae Herba can be effectively applied to the treatment of rheumatoid arthritis and it is still necessary to isolate effective compound from Siegesbeckiae Herba in the near future.