Objectives : The present study was undertaken to determine whether the water extract of Achyranthis Radix, which is the roots of Achyranthes japonica (Achyranthis Radix, AR), is efficacious against collagen-induced arthritis (CIA) in DBA/1J mice. Methods : Mice were immunized with bovine type II collagen and orally treated with AR-W (50 and 100 mg/kg/bw) from days 21 to 42 after immunization. Arthritis was evaluated by arthritic score, histological examination of knee joint and serological markers such as TNF-${alpha}$, $PGE_2$ and anti-type II collagen (C II)IgG. Results : The results showed that comparing with untreated CIA mice, treated with AR-W significantly suppressed the clinical score and joint tissue pathological damages, reduced the serum levels of TNF-${alpha}$, $PGE_2$ and anti-C II IgG in CIA-mice. These results suggest that AR-W can effectively alleviate inflammatory response on CIA, and anti-inflammatoy of AR-W can be attributed, at least partially, to the inhibition of inflamamtory mediators, $PGE_2$ and TNF-${alpha}$, in CIA. Conclusions : This study suggests that AR-W has a therapeutic potential in inflammatory joint diseases such as rheumatoid arthritis.