We report an extremely rare case of 42-year-old man, d iagnosed with thymic dedifferentiated liposarcoma (DDL) having a leiomyosarcomatous dedifferentiated component. The tumor was composed of atypical spindle cells. The morphological atypia was also observed around thymic adipose tissue. Immunohistochemically, the spindle cells were positive for desmin, smooth muscle actin, MDM2 and CDK4. MDM2 and CDK4 were also positive for the thymic adipose tissue. MDM2 amplification was confirmed by fluorescence in situ hybridization. Although there are some reports of mediastinal leiomyosarcoma, DDL may be overlooked. For accurate diagnosis of mediastinal mesenchymal tumor, careful observation of thymic adipose tissue is required with immunohistochemical study.