We performed a revisionary open reduction and internal fixation for treating nonunion of the mid-shaft of the left clavicle with an autogenous cancellous bone graft. On postoperative day 4, the patient presented with neurologic deficits in the left upper extremity. We removed the implant and made a superior angulation to decompress the brachial plexus. At 6 months postoperatively, callus bridging and consolidation were visible and all hand and elbow functions were fully recovered. Our case suggests that brachial plexus neuropathy may be caused by stretching and compression after reduction and straightening of the nonunion site around adhesions or scar tissue. Therefore, care should be taken whether there are the risk factors that can cause brachial plexus neuropathy when revision surgery is performed for treating nonunion of a clavicle shaft fracture.