Objective To investigate the imaging features of Ewing's sarcoma and primitive neuroectodermal tumor (ES and PNET) in sacrum.Methods Imaging data were retrospectively collected and reviewed in 17 patients with ES and PNET confirmed by pathology.X-ray was performed in all patients,CT scans in 13 patients,and MR scans in 10 patients.Results All lesions were detected solitary and lateral.X-ray and CT features were lytic bone destruction in 12 cases,2 of them with bone sclerosis; one patient showed diffuse sclerosis.Soft tissue mass in pelvic cavity can be seen in 11 cases,9 of them with adjacent large blood vessels invasive and 4 patients with sacroiliac joints invasive.On post-contrast images,mild enhancement was found in 8 cases,and 5 patients with marked enhancement.MR images showed intermediate signal intensity on T1WI in 8 cases and heterogeneous moderate to high signal intensity on T2WI/fat suppression in 10 cases.DWI showed high signal intensity in 4 cases.Unlike CT scan,contrastenhanced MR images showed significant enhancement in 7 cases.Conclusions Ewing's sarcoma and primitive neuroectodermal tumor (ES and PNET) in sacrum have some specific imaging features.Imaging examination could provide reliable evidence for clinical diagnosis.