Objective：To explore the possible etiopathogenesis，diagnosis and treatment for malignant giant cell tumor of bone （MGCT）. Methods：Six cases from January 2001 to December 2012 were chosen，who had been clearly diagnosed as MGCT and were followed up successfully. Their clinical data were analyzed and relevant literatures were reviewed. Results：The six cases were all secondary MGCT. Average internal time from initial benign diagnosis to malignant transformation of giant cell tumor of bone was 52.7 months. Histopathologically，three cases presented with fibrosarcoma，two with osteosarcoma and one with malignant fibrous histiocy－toma. Treatments included radical surgeries for three，margin excision with adjunctive therapy for one，high intensity focus ultrasound therapy for one and chemotherapy with palliative treatment. Mean follow-up time was 22.8 months，two cases had no evidences of dis－eases，two were alive with tumors and two died. Conclusions：MGCT accounts for approximately 5％ of the total cases of giant cell tu－mor of bone（GCT）. Radiotherapy and recurrent incomplete curettage might induce malignant transformation. Chromosome instability，heteroploid appearance and centrosome aberration might play an important role in the malignant transformation. GCT displaying local recurrence over 3 years after the initial treatment should be considered the possibility of becoming malignant. Early detection and radi－cal surgery might improve the prognosis.