Objective To investigate the MRI features and Ki-67 expression and correlation with prognosis in undifferentiated pleomorphic sarcoma (UPS).Methods MRI and clinicopathological data of 34 cases with UPS proved by operation and pathology were analyzed retrospectively(with an average follow-up of 3 years).According to the presence of local recurrence,the data were divided into two groups:no recurrence group (15 cases)and recurrence group(19 cases).Two groups were analyzed according to age,tumor size,location,MRI features (lesions margin,hemorrhage,necrosis,pseudocapsule and perilesional characteristics)and expression of Ki-67.Results The results showed that two groups tumor’s size(P =0.002),location(P =0.025),pseudocapsule(P =0.025 ),peritumoral tissue inva-sion(P =0.008)and expression of Ki-67 (P =0.000)were of statistical correlation with tumor local recurrence.Conclusion Preoper-ative MRI can observe the characteristics of undifferentiated pleomorphic sarcoma,the relationship with the adjacent tissue and local invasive scope,which will be helpful for clinical diagnosis and decision of surgical plan.