Ionizing radiation has brought powerful aids to medical practitioners, but it has also brought new and bewildering problems. Radiation in the form of systemically administered radionuclides or external radiation may produce such abnormal reactions in the musculoskeletal system as growth disturbances, myelofibrosis, radiation osteitis, necrosis of bone, exostosis, pathologic fracture, and on rare occasion malignant tumors. The incidence of postirradiation sarcoma was quite rare and there were some histologic hypes of postirradition sarcoma. Of them, incidence of osteosarcoma and fibrosarcoma is most common, although there is some difference between each reports. The authors have experienced recently two cases of postirradiation fibrosarcoma developed in sacrum. In one case, the patient was 55 years old female and the latent period was about 12 years. In another case, the patient was 67 years old female and the latent period was about 15 years. In these cases, they received radiation therapy due to cervical cancer of uterus. They had nerve roots compression symptoms and recieved operation of nerve roots decompression. The diagnosis was confirmed by core bone biopsy. A clinical analysis of these cases is to be reported with review of references.