The most common type of soft tissue sarcomas is liposarcomas. It consists of 0.1-0.2% of all malignant tumors in adulthood. Retroperitoneal liposarcomas are generally seen in individuals between 40-60 years old, however are also seen in younger ages. MRI provides important information about localization of mass, tumor infiltration and relationship between vascular structures before the operation. In addition, CT and MRI provide reliable data about early diagnosis of local recurrence at follow-up. Aggressive surgical resection is the treatment option for extending survival. Nephroureterectomy should be added to aggressive resection. Due to high rates of local recurrence, patients should be followed-up by CT and MRI in first two years quarterly, then once in every 6 months per following 3 years. Here, we presented a case as follow; 50 years old male who complained with left flank pain and diagnosed as dedifferentiated liposarcomas.