Rupture of the patellar tendon is relatively rare. We report a case of patellar tendon avulsion with a tibial tuberosity sleeve fragment in pediatric patient. In pediatric patient, diagnosis is sometimes difficult due to uncompleted ossification. In the present case, which involved the presence of a small fleck of bone from tibia, we were able to attain a diagnosis using the Koshino-Sugimoto index and MRI and easily determine the optimal treatment with the use of the suture anchor and tension band wiring method.