Fibrous dysplasia (FD) is generally considered to be a benign disease that affects the bones, but it has potential to become malignant over time, generally several decades after its initial diagnosis. Radiation therapy can induce malignant transformation of FD; however, reports have indicated a few cases of malignant transformation of FD in the absence of radiation therapy. Angiosarcoma is a particularly rare type of cancer in the oral region, which accounts for less than 1% of all soft-tissue sarcomas. Herein, we reported a case of a 62-year-old man with monostotic FD of the left maxilla of over 50 years' duration that underwent malignant transformation into an epithelioid-type angiosarcoma. To the best of our knowledge, this is the first report of such case.