A malignant tumor from the thigh of a 65-year-old man was examined by light and electron microscopy. The tumor was diagnosed by light microscopy as malignant mesenchymoma with prevalence of leiomyosarcomatous or liposarcomatous components, but electron microscopic study revealed that the basic cell was similar to chondroblastic or osteoblastic cells. However, a precise ultrastructural classification of the tumor cells was not possible, since the majority of the cells had mixed features. The histogenesis, differentiation, and relation to prognosis are discussed.