Background. MZB is a purine analog, and is used as a disease modifying anti-rheumatic drug (DMARD). We conducted an open label uncontrolled clinical trial to evaluate the efficacy and safety of combination therapy with methotrexate (MTX) and mizoribine (MZB). Methods. Thirty one RA patients (9 males, 22 females, 68±12 year-old) who fulfilled ACR criteria of RA and did not show sufficient clinical response to MTX were included. MZB (150 mg/day, once a day) were added to MTX. DAS28-CRP was measured at day 0 and 1, 3, 6, and 12 months after the treatment. Adverse events were recorded. Results. Overall DAS28-CRP was significantly decreased from 4.4±1.0 to 3.1±1.3 at 3 months (p<0.01), 2.7±0.68 at 6 months (p<0.01), 2.4±1.4 at 12 months (p<0.01). Seventeen patients (55%) achieved significant improvement of DAS28-CRP. Number of swollen joints of responders before the treatment was significantly fewer than that of non-responders. Improvement of DAS28-CRP was significantly different between the responders (0.91±0.74) and non-responders (0.18±0.66) at 1 month (p<0.01). Nine patients (29%) could achieve remission Four patients experienced adverse events. Conclusions. MTX and MZB combination therapy was effective and relatively safety.