Objectives To study the expression of IL-17 and RANKL in synovium of CIA rat.To explore the influence of 99Tc-TDP on IL-17 and RANKL and the treatment mechanisms of 99Tc-TDP.Methords The rats were induced by injecting of typeⅡcollagen and complete Freud's adjuvant. The rats successfully induced CIA(n=20) were divided into two groups randomly: the model group and the 99Tc-TDP group(0.05μg/kg/d99Tc, tail vein injection). The blank group and the model group were treated with saline tail vein injection. Arthritis were evaluated by arthritis index (AI), the serum and synovial fluid concentration of IL-17 and RANKL, the histopathological score, the expression of IL-17 and RANKL in synovium detected by immunohistochemistry. Results The arthritis index, the serum and synovial fluid concentration of IL-17, the synovial fluid concentration of RANKL, the expression of IL-17 and RANKL in synovium increased in CIA rats（P<0.05）. The 99Tc-TDP treatment prevented the increase of all above indexes（P<0.05）.The serum concentration of RANKL of all the groups was low(P>0.05). Conclutions IL-17 and RANKL play an important role in the synovium lesions of CIA rat.99Tc-TDP can relieve the arthritis syndrome and retard the bone erosion and the mechanisms are at least in part inhibiting IL-17 and RANKL production.