Objective To determine the accuracy of endorectal ultrasonography (ERUS) in preoperative staging of rectal cancer and investigate the limitations and pitfalls of ERUS. Methods Ninety-four patients with rectal cancer were examined preoperatively by ERUS between September 2008 and November 2009 in this hospital. The size, shape, echo pattern, infiltration depth, and extra-rectal invasion of lesions were observed. The results of ERUS staging were compared with histopathology findings of the resected specimens. Results The overall accuracy of ERUS in T staging was 63.8% (60/94). The accuracies of ERUS for pT1, pT2, pT3, and pT4 tumor were 87.2% (82/94), 76.6% (72/94), 76.6% (72/94), and 97.9% (92/94), respectively. The sensitivity, specificity, and accuracy of ERUS for advanced rectal cancer (pT3+pT4) were 70.8% (34/48), 78.3% (36/46), and 74.5% (70/94), respectively. The sensitivity, specificity, and accuracy of ERUS in lymph node metastasis were 75.0% (42/56), 42.1% (16/38), and 61.7% (58/94), respectively. There was no significant difference of accuracy among various tumor locations above anocutaneous line (P＝0.495). The accuracy of ERUS for T staging improved with experience, the T staging accuracy improved from 40.0% after assessment of 30 cases to 81.3% after 94 cases were examined (P＝0.026). Conclusions The ERUS provides a good accuracy rate for assessment of the depth of tumor invasion and lymph node metastasis of rectal cancer, and has become an important imaging tool for preoperative staging rectal cancer. The operator experience, peritumoral inflammation mainly influences the accuracy of ERUS.