Liposarcoma is one of the most common soft-tissue sarcomas and classified as five groups. Dedifferentiated liposarcoma includes multiple elements such as solid tissue, adipose tissue and calcified tissue, which is different from common liposarcoma in tissue constitue. Dedifferentiation to osteosarcoma is rarely described in the literature. We reported the radiological and pathological findings in a case of dedifferentiated liposarcoma with osteo-sarcomatous dedifferentiation in the retroperitoneum and discussed the characteristic features of this specific tumor. Complete surgical resection of the neoplasm is the only curative means, but establishing a prognosis remains a challenge for long-term evaluation.