A 64 year-old female visited our outpatient clinic with left dull flank pain. The ultrasonography and CT scans showed the large solid mass in 12 cm in maximum diameter, containing large area of fatty element, in the left retroperitoneal space. Under the diagnosis of retroperitoneum liposarcoma, the mass and left kidney was completely removed. Pathological examination revealed the dedifferentiated liposarcoma which contained the distinct two patterns of well differentiated lipoma and dedifferentiated fibrotic sarcomatoid tissue.