Objectives: Calpain, a calcium-dependent cysteine proteinase, may be one of the proteolytic enzymes that mediate cartilage degradation associated with rheumatoid arthritis. The object of this study is to ascertain immunohistochemically whether calpain is present in the inflamed joints of collagen-induced arthritis of rats, and examine the effect of Cortex Acanthopanacis Senticosi on the expression of calpain. Methods: Male Lewis rats, around 200g of body weight, were immunized with bovine type II collagen. After 3 weeks from first immunization, rats were divided into arthritic control (n=6) group and Cortex Acanthopanacis Senticosi-treated (n=6) group. Non-immunized rats served as the normal (n=6) group. All animals were sacrificed at 15 days post-treatment and tibiotarsal joints were removed. Calpain immunohistochemistry was performed on the midsagittal section of the tibiotarsal joint. Results: All animals of the control and treated groups showed ankylosing osteoarthritis. However, the animals of the treated group showed alleviation in the fibrous ankylosis, destruction of articular cartilage and destruction of subchondral bony tissue compared with the animals of the control group. Calpain was expressed in the chondrocyte lacunae of growing articular cartilage, in the skeletal muscle fibers, in the peripheral nerves, and in the vessel walls around the joints of all groups. In the control and treated groups, calpain was also expressed in proliferating synovial epithelia, subsynovial stroma cells, surface of articular cartilage, and fibrous pannus around destructive subchondral bony tissue. However, the expression density of calpain in the treated group was diminished compared with the control group, especially in surface of articular cartilage and fibrous pannus. Conclusions: These observations indicated that calpain plays an important role in the destruction of cartilage and bone in collagen-induced arthritis of rats, and also indicated that Cortex Acanthopanacis Senticosi inhibits the development of arthritis by decreasing the expression of calpain.