Objective: To investigate the effects of biological disease-modifying antirheumatic drugs (bDMARDs) on diabetes control among patients with rheumatoid arthritis (RA).Methods: A total of 296 patients with RA were included in the study. The following background factors were investigated: age, gender, bDMARD type, methotrexate and prednisolone (PSL) dosages, glycated hemoglobin (HbA1c), C-reactive protein, and matrix metalloproteinase-3. We used the simplified disease activity index (SDAI) to evaluate the RA disease activity. Poor diabetes mellitus (DM) control was defined as a HbA1c of 6.0; accordingly, the patients were divided into good and poor DM control groups. SDAI and PSL dosage were the primary endpoints, respectively, 1 year later. Results: HbA1c ranged from 6.6±0.68 to 6.5±0.82 and 5.1±0.29 to 5.4±0.34 in the poor and good DM control groups, respectively. Although the intergroup difference was significant (p=0.000), there was no significant intergroup difference during the treatment period (p=0.084). The SDAI ranged from 27.7±15.6 to 7.1±8.0 in the group with a poor DM control (n=83) and from 22.9±14.0 to 6.3±7.6 in the group with a good DM control (n=213).Conclusion: The bDMARD therapy reduced the RA disease activity regardless of a good or poor DM control.