Osteoblastoma is a rare, bone-forming tumor, characterized by osteoid and woven bone production. A 13-year old boy patient presented to our clinic with complaint of pain in his left proximal tibia. We performed curettage and bone grafting for the lesion diagnosed as osteoblastoma. Two years later, the patient admitted to the hospital with a mass in the same region which was diagnosed by biopsy to be osteosarcoma. Patient was performed reconstruction operation with local resection and mega prosthesis. Fourteen months after termination of chemotherapy, lung metastasis developed and the patient died consequently. In this article, we reported a patient with aggressive osteoblastoma of the left proximal tibia which recurred as an osteosarcoma and discussed the difficulties in the histopathological diagnosis and management of these patients. As some other cases in the literature, our case indicates that osteoblastomas may undergo malignant transformation.