Lipomas are the most common benign tumors of adipose tissue among adults. Lipomas can occur almost anywhere in the trunk, extremities, mediastinum, and pelvis, but retroperitoneal lipomas are extremely rare. It should be distinguished from well differentiated liposarcoma in order to provide the appropriate treatment and follow up. We experienced a case of 60-year-old patient with large retroperitoneal lipoma mimicking liposarcoma causing palpable abdominal mass and pain. Abdominal computerized tomography (CT) showed 33×22 cm sized bulky fat-containing mass with contrast enhanced solid portion in right retroperitoneum. Positron emission tomograpgy (PET) revealed increased 18F-FDG uptake at solid portion shown in abdominal CT. Imaging studies confirmed a high index of suspicion on liposarcoma. Laparotomy showed a large encapsulating tumor arising from retroperitoneum with fat necrosis. Pathologic examination of resected specimen revealed normal mature adipocytes without atypical cells, compatible with lipoma.