This report describes a 6 year-old boy who was treated with one-bone forearm procedure for acquired pseudoarthrosis of the ulna combined with radial head dislocation after radical ulna debridement for osteomyelitis. At more than 20 years of follow-up, the patient had a nearly full range of elbow movements with a few additional surgical procedures. Pronation and supination was restricted by 45°, but the patient had near-normal elbow and hand functions without the restriction of any daily living activity. This case shows that one-bone forearm formation is a reasonable option for forearm stability in longstanding pseudoarthrosis of the ulna with radial head dislocation in a child.