Objectives: To investigate the value of three-dimensional Endorectal ultrasound in evaluating complete pathologic response after preoperative neoadjuvant chemoradiation therapy for locally advanced rectal cancer. Methods: 98 patients with locally advanced rectal cancer in our hospital were retrospectively analyzed. All patients underwent total mesorectal excision after neoadjuvant chemoradiation therapy. 3D-Endorectal ultrasound was performed within one week of first admission and one week after preoperative neoadjuvant chemoradiation therapy. The correlation between each parameter and TRG grade was analyzed by Spearman correlation method; ROC curve was used to evaluate the diagnostic efficacy of the parameters. Results: There was a positive correlation between involved length（r=0.336），thickness（r=0.403）

intestine（r=0.339） and TRG grade (all P<0.05)

a negative correlation between the length change rate（r=-0.407）

the thickness change rate（r=-0.435）

the invasion of intestine change rate（r=-0.408） and the blood flow change rate（r=-0.235） and TRG grade after neoadjuvant chemoradiation therapy (all P<0.05).The ROC curve shows the diagnostic performance 3D-ERUS predictive pCR

including the cut-off value of the length change rate is 29％

the sensitivity and specificity is respectively 70.0％

76.9％

the thickness change rate is 44%

the sensitivity and specificity is respectively 80.0％

76.9％

the invasion of intestine change rate is 33%

the sensitivity and specificity is respectively70.0％

79.5％

the blood flow change rate is 0％

the sensitivity and specificity is respectively70.0％

57.7％

the cut-off value of the involved length was 39 mm

the sensitivity and specificity is respectively95.0％

39.7％

the involved thickness was 10 mm

the sensitivity and specificity is respectively100.0％

47.7％

the involved intestine was 25％

the sensitivity and specificity is respectively70.0％

70.5％ after neoadjuvant chemoradiation therapy. Conclusion: 3D-Endorectal ultrasound is of great value in the assessment of complete pathologic response after neoadjuvant chemoradiation therapy for local advanced rectal cancer. The involved length

thickness and intestine

the length change rate

the thickness change rate

the invasion of intestine change rate and the blood flow change rate after neoadjuvant chemoradiation therapy measured on 3D-Endorectal ultrasound present high accuracy in prediction of complete pathologic response.