Background: Although a brachial plexus block can be used to provide anesthesia and analgesia for upper extremity surgery, its effects using $MgSO_4$ on postoperative pain management have not been reported. The aim of this study was to evaluate brachial plexus block using $MgSO_4$ on postoperative analgesia. Methods: Thirty-eight patients who were scheduled to undergo upper extremity surgery were randomly allocated into two groups: patients receiving axillary brachial plexus block with 0.2% ropivacaine 20 ml and normal saline 2 ml (group S) or 0.2% ropivacaine 20 ml and $MgSO_4$ 200 mg (group M). Before extubation, the blocks were done and patient controlled analgesia was started, and then, the patients were transported to a postanesthetic care unit. The postoperative visual analogue scale (VAS), opioid consumption, and side effects were recorded. Results: The two groups were similar regarding the demographic variables and the duration of the surgery. No differences in VAS scores were observed between the two groups. There was no statistically significant difference in opioid consumption between the two groups. Nausea was observed in three patients for each group. Conclusions: Axillary brachial plexus block using $MgSO_4$ did not reduce the level of postoperative pain and opioid consumption.