This study aimed to investigate the anti-inflammatory, anti-arthritic and immuno-regulatory effects of electro-acupuncture (EA) at ST36 on Collagen-induced arthritis (CIA) in mice. Male DBA/1J mice were divided into five groups: Normal, Control, NR (needle retention), EAI and EAII. All mice except those in the normal group were immunized with Collagen II for arthritis induction. Acupuncture needles were inserted into mice ST36 and electrical currents at a frequency of 2 Hz in a continuous rectangular wave form were conducted through the needles for 15 min, 3 times a week. EA treatments were administered for 5 weeks in the EAI group and for 9 weeks in the EAII group. The mice in the NR group were acupunctured in the same manner as the EA groups and the needles were retained for 15 min without electrical stimulation. CIA incidence analysis, ELISA, histological analysis and FACS analysis were performed to evaluate the effect of EA on CIA. EA at ST36 significantly reduced CIA incidence, IL-6, TNF-a, INF-γ, collagen II antibody, IgG and IgM levels in CIA mice serum and prevented knee joint destruction. EA at ST36 also reduced CD69+/CD3e+ cells and CD11a+/CD19+ cells in CIA mice lymph nodes, and CD11b+/Gr1+ cells in CIA mice knee joints. The ratios of CD3e+ cells to CD19+ cells, and CD8+ cells to CD4+ cells were maintained closer to the normal range in the EA groups as compared with the control group or the NR group. EAII was more effective than EAI throughout all the measurements. The NR was effective as well, though less effective than EA. EA at ST36 may have an anti-inflammatory, anti-arthritic and immuno-regulatory effects on CIA in mice. The effectiveness is stronger when EA starts earlier and is applied longer. Needle retention without electrical stimulation may be effective on CIA as well, however less effective than EA. Electrical stimulation and acupoint ST36 may have synergistic effects on CIA.