Objective To investigate the effects of melittin on Th17/ Treg balance and arthritis-associated inflammation in a rat model of collagen-induced arthritis. Methods Thirty Sprague Dawley rats were randomly allocated into a normal group, while 24 SD rats were injected with bovine collagen II on the 1st day and again on the 10th day to establish a collagen-induced arthritis model. On the 14th day, elimination of 6 unmodeled rats and 18 model rats in which the model had been successfully established were selected and divided into a model group, a melittin group, and a methotrexate group by a random number table method. On the same day, the melittin group was injected with melittin (0.5mg/ kg, once every other day), while the methotrexate group was injected with methotrexate (1 mg only, once per week,three times). The model group and the normal group were intraperitoneally injected with equal amounts of distilled water;then, on the 32nd day, the rats were executed. The concentrations of TNF-α, IL-10, and IL-17 A in serum were determined by ELISA. The spleen of each rat was removed, the leukomonocytes were extracted, the activity of leukomonocytes was detected by MTS, and the proportions of Th17 and Treg cells were determined by flow cytometry.Pathological changes of the ankle joint were also observed by HE staining. Results Melittin and methotrex significantly decreased the concentrations of TNF-α and IL-17 A ( P < 0. 01, P < 0. 05), increased the concentration of IL-10 ( P <0.05), inhibited the activity of leukomonocytes ( P < 0. 05), decreased the proportion of Th17 cells ( P < 0.05), and increased the proportion of Treg cells ( P < 0. 01, P <0. 05). In the melittin group and methotrex group, the proliferation of synovial membrane was decreased, which prevented formation of the pannus and delayed the destruction of bone and articular cartilage. Conclusions Melittin can regulate Th17/ Treg balance and inhibit joint inflammation in rats with collagen-induced arthritis.