Smooth muscle tumors are very common tumors in the uterus and related adjacent structures but occur rarely in the retroperitoneum. Traditionally, most retroperitoneal smooth muscle tumor are believed to be malignant. But well-differentiated smooth muscle tumors with lack of atypia, necrosis, and significant mitotic activity appear to have a benign behaviors. Laparotomy revealed a huge solid tumor in the retroperitoneal space, about 50 cm in diameter, and histologically diagnosed as a smooth muscle tumor of uncertain malignant potential (STUMP). We report a case of primary retroperitoneal smooth muscle tumor with a brief review of literatures.