PURPOSE: We analyzed the oncologic outcome of the malignant transformed benign giant cell tumor of bone. MATERIALS AND METHODS: Between January 2000 and February 2012, 5 cases were referred with suspicious malignant transformation of benign giant cell tumor. No patients underwent radiation therapy. RESULTS: After referral, all patients received the wide excision of the tumor and its' pathologic diagnosis were osteosarcoma. As classified by the location of tumor lesion, 3 cases were located in the distal femur, 1 case was in the distal radius and 1 case was in the proximal femur. The average latent period between diagnosis of benign giant cell tumor and diagnosis of secondary malignant giant cell tumor was 49.2 months. (range, 24-126 months) The mean follow-up period was 21.6 months. There were subsequent local recurrence in 2 cases and 3 patients developed distant metastasis. All patients with lung metastasis were dead. CONCLUSION: Malignant transformation of benign giant cell tumor of bone can be occurred within 5 years. Therefore, when benign giant cell tumor suspicious malignant transformation, it is necessary to do more aggressive treatment.