Abstract: Small cell undifferentiated Carcinoma of Rectum Was uncommon finding. It has been reported rarely at abroad and noreported at home. This disease accounted for Less than 1% of malignant tumor of rectum. The author report 2 patients with small cell undifferentiated carcinoma of rectum which proven by pathological examination. This paper deals with clinical behavior, diagnosis,pathology, etiology, treatment and prognosis of this disease. It is to emphasize that the patientShould be given active combined treatment including operation, radiation therapy and chemotherapy.