Radioulnar synostosis which develops after treatment of isolated olecranon fracture is a rare complication. The aim of this study was to determine the clinical findings and postoperative outcomes of radioulnar synostosis after isolated olecranon fracture in a child patient. A 14year-old girl was evaluated after falling on her left elbow. She had pain, edema, and motion limitation in her left elbow. After radiologic examinations, diagnosis of olecranon fracture was made. Olecranon fracture was fixated by open reduction and internal fixation with tension band wiring method via the posterior approach. When the patient came to the control to remove the implants 9 months after the first operation, there was a limitation in the supination and pronation movements. In the radiographs, synostosis was observed in the proximal region between the radius and ulna. The patient was reoperated to remove the implants. In the same session, synostosis was excised by using the posterior approach, and a barrier between the bones was constituted with bone wax and early elbow range of motion exercises started. In the postoperative first month, the patient had full flexion and extension but with 30 degrees of supination deficit. Radioulnar synostosis is rare but can be seen after isolated olecranon fractures. Early elbow motion after radioulnar synostosis surgery helps the patient to increase joint movement.