Osteosarcoma in soft tissue is rare and its prognosis is very grave. Comparing with the osteosarcoma of bone, the former is rarer in incidence, poorer in prognosis and found in older patients. Trauma, irradiation or myositis ossificans are considered to be the causes of this tumor. The radical and meticulous surgery is indispensable to eradicate the focus, but there is not infrequent local recurrence and metastasis. We experienced a case of osteosarcoma in the calf muscle, which seemed apparently to have developed from the myositis ossificans without any definite trauma history. In this case the tumor focus was controlled with surgery and radiation therapy but it was found metastasized to the lung and scalp afterwards.