Methods: Patients, who were diagnosed with ultra-low rectal cancer and had strong desire of preserving anal function, will take part in the research,whose tumor invading the levator ani muscle or above, the diameter less than 6cm, the lower tumor margin away from the anal verge by less than 5cm and away from the dentate line by less than 3cm.They will undergo total mesorectal excision(TME) and ISR combined with IORT and prophylactic ileostomy.