We report two cases of retroperitoneal liposarcoma arisen from the perirenal fat tissue, which could not be diagnosed preoperatively. Case 1 is a 58-year-old male. He complained of left flank tumor. Computed tomography and magnetic resonance image showed a mass over 10 cm that contained fat components in the retroperitoneal space. The tumor was resected with left nephrectomy and histological examination revealed well differentiated liposarcoma. As adjuvant therapy, he received chemotherapy and 30 months has passed uneventfully. Case 2 is a 70-year-old male. Screening ultrasonography revealed incidental retroperitoneal tumor. With clinical diagnosis as non-functioning adrenal tumor, he received left nephrectomy. The pathological diagnosis was well differentiated liposarcoma, sclerosing type. No adjuvant therapy was performed. He has stopped visiting our clinic due to aggravation of heart disease. The characteristics of the images of the two cases were different despite the histological resemblance. This difference was considered to be due to the difference in the distribution of lipomatous tissue in each patient.