OBJECTIVE：To study the effec ts of Tibetan Codonopsis tralictrifolia extract （called“ZDS”for short ） on collagen-induced arthritis （CIA）model rats and its mechanism. METHODS ：Eight of 48 rats were randomly selected as normal control group （normal saline ），and the remaining 40 rats were used to establish CIA model. After successful modeling ，the rats were randomly divided into model group （normal saline ），ZDS low-dose ，medium-dose and high-dose groups （0.44，0.88，1.76 g/kg，by crude drug ），dexamethasone group （positive control ，0.002 5 g/kg），with 8 rats in each group. They were given relevant medicine intragastrically ，the volume of 400 μL，once a day ，for consecutive 28 days. The body weight of rats were weighed before medication （0 d），7，14，21 and 28 days after medication ；and arthritis indexes were scored. The pathological changes of the knee joint synoviual tissue were observed after last medication. The thymus index ，spleen index ，the levels of serum inflammatory factors （IL-1β，TNF-α，IL-6），protein expressions of NF-κB p65，p-NF-κB p65，IκB and p-IκB in synovial tissue were detected. RESULTS ：Compared with normal control group ，the body weight （14，21，28 days after administration ）of rats in model group was significantly reduced （P＜0.05）；the arthritis index score （before administration and different administration time）was significantly increased （P＜0.05）；the joint synovial tissue was pathologically damaged ；the thymus and spleen index ， inflammation factor level ，the protein expression of p-NF-κB p65 and p-IκB were increased significantly（P＜0.05），while the protein expression of IκB was decreased significantly（P＜0.05）. Compared with model group ，the level of IL- 1β was decreased significantly in ZDS low-dose group （P＜0.05）. Body weight of rats （21，28 days after administration ）were increased significantly in ZDS medium-dose and high-dose groups ，dexamethasone group （P＜0.05），while arthritis index score （14，21，28 days after administration ） was decreased significantly （P＜0.05）. The pathological damage of joint synovial tissue was significantly relieved ；thymus and spleen index ，inflammation factor level ，the protein expression of p-NF-κB p65 and p-IκB were significantly reduced，while the protein expression of IκB was significantly increased （P＜0.05）. CONCLUSIONS ：ZDS can improve CIA model rats to some extent ，and its mechanism may be related to the inhibition of NF-κB signaling pathway.