We present a rare case of malignant transformation of desmoplastic fibroma in a 37-year-old man. In 1984 we performed curettage and bone grafting for a bone tumor of the left distal femur. Histologically, we obtained a final diagnosis of desmoplastic fibroma. After this treatment, the patient had no particular symptoms but felt tension in the left knee in 2000 and consulted our department. We performed tumor curettage and bone grafting although malignant findings were recognized by imaging. The last pathology diagnosis was an osteosarcoma (in view of the formation of an osteoid). We performed caffeine-assisted chemotherapy and an additional wide excision. In the literature, two reports of a high-grade sarcoma developing in a desmoplastic fibroma have appeared. It is sometimes difficult to distinguish desmoplastic fibroma from low-grade fibrosarcoma or intraosseous-type osteosarcoma, although the radiographic findings and aspects of the resected specimen in 1984 were typical of a desmoplastic fibroma. Moreover, from a clinical standpoint, because the recurrence came 16 years after the initial operation, and lung metastasis was not seen, it is difficult to believe this case was malignant originally. This extremely rare lesion is believed to be a desmoplastic fibroma transformed to osteosarcoma.