Objective To investigate the value of the three-dimensional endorectal ultrasonography (3D-ERUS) in the tumor staging before transanal endoscopic microsurgery (TEM) . Methods The clinical data of 30 patients with rectal cancer who underwent 3D-ERUS before TEM at the Nanjing Hospital of Traditional Chinese Medicine from April 2012 to December 2013 were retrospectively analyzed. The accuracy, sensitivity and specificity of the 3D-ERUS were evaluated according to the results of the postoperative pathological examination. The consistency of the results of the 3D-ERUS and postoperative pathological examination were compared by Kappa consistency test. Results Of 30 patients, 25 patients in stage T0, 3 in stage T1 and 2 in stage T2 were diagnosed by preoperative 3D-ERUS. There were 2 patients (stage pT0) with inflammatory polyp by postoperative pathological diagnosis, 6 patients (stage pT0) with tubular adenoma, 16 patients (stage pT0) with villioustublar adenoma, 2 patients (stage pTis) with carcinoma in situ, 2 patients (stage pT1) with rectal adenoma and 2 patients (stage pT2) with rectal adenoma. There were 2 patients with excessive tumor staging by 3D-ERUS, 1 patient in stage pT0 was misdiagnosed in stage T1 , 1 in stage pT1 was misdiagnosed in stage T2 and 1 in stage pT2 was misdiagnosed in stage T1 with insufficient tumor staging. The accuracy of 3D-ERUS in the preoperative tumors staging of TEM was 90.0% compared with the resuls of postoperative pathological examination. The accuracy, sensitivity and specificity of 3D-ERUS in stage pT0, pT1 , and pT2 of TEM were 96.7% , 90.0% , 93.3% and 96.2% , 50.0% , 50. 0% and 100.0% , 92.8% , 96.4% , respectively. There was a significant difference in the consistency between preoperative 3D-ERUS and postoperative pathological examination (κ =0. 685, P <0.05) . Conclusion 3D-ERUS is an accurate clinical method in the preoperative tumors staging of TEM, and can be used as the preoperative assessment for TEM. Key words: Rectal neoplasms; Three-dimensional endorectal ultrasonography; Tumor staging; Microsurgery