Abstract: Objective To evaluate the accuracy of endorectal ultrasound （ERUS） in the TN staging of rectal cancer after neoadjuvant chemoradiation in comparison with postoperative pathology. Methods Forty patients with rectal cancer were chosen in our hospital from January 2013 to December 2013. All patients received dimensional conformal radiotherapy or intensity modulated radiotherapy （DT50Gy／25 fractions）. From the first day of radiotherapy， S-1 capsules were administered orally for two periods which including two weeks administration and one week drug withdrawal. Operations were carried out in six to eight weeks after chemoradiation. Patients were given ERUS before operation and postoperative pathology. Stages of patients before and after operation were compared. Results The diagnosis accuracy rates of ERUS for T1， T2， T3 and T4 stage were 77.8％， 76.9％， 83.3％ and 83.3％， respectively. The total diagnosis accuracy rate of T stage was 80.0％. The diagnosis accuracy rates of endosonography for N0 and N+ stage were 88.4％ and 62.5％， respectively. The total diagnosis accuracy rate of N stage was 80.0％. The total diagnosis accuracy rate of TN was 72.5％. Conclusion ERUS has good accuracy in T stage and N stage diagnosis. It is one of the most common used detective methods in preoperative diagnosis of rectal cancer.