A humero-ulnar synostosis is a bony connection between the humerus and the ulna. This is a very rare finding and it results in a serious disability of the elbow. Usually, a synostosis of the elbow occurs as a congenital anomaly. In this case, a 6-year-old girl was seen with a post-traumatic humero-ulnar synostosis, which has never been reported in the literature before. Surgical resection of the humero-ulnar synostosis was performed. Along with rapid intensive physical therapy, almost full recovery of function was achieved. The short-term result is very satisfactory, but the long-term results and recurrence rate are still unknown.