Abstract： Objective To investigate the occult lymph node metastasis in the middle and lower third rectal cancer after neoadjuvant radiotherapy. Methods From June 2003 to December 2006, 74 rectal cancer patients received neoadjuvant radiotherapy (30Gy/10 f/2w, CACA-CRC-001) and underwent total mesorectal excision (TME) two weeks later. Fat clearance technique was used in all the samples. Occult lymph node metastasis was detected in the mesorectum using the anti-CK antibody.Results In total 1883 retrieved lymph nodes, 172 metastasis lymph nodes were harvested by HE examination with the mean diameter [(4.9±2.6) mm]being larger than that (2.7±1.4) mm of the 1711 negative nodes (P<0.01). In HE negative nodes, occult metastasis was found in 40 lymph nodes (2.33%) from 24 patients. Most of these nodes were less than 5 mm(90.0%) with a mean diameter of (3.2±1.2)ram, smaller than those of HE-positive metastasis nodes (P<0.01). Occult metastasis was found in 23.1% (9/39) of HE-negative patients. Occult metastasis incidence was higher in patients with HE-positive nodes(42.8% ,P<0.01). No correlation of lymph node occult metastasis with tumor differentiation,age, or surgical procedures was found. There was no significant difference in recurrence-free survival between ypN_0 patients with and without occult metastasis(P=0.157). Conclusion It is not necessary to include occult lymph node metastasis in the TNM staging in patients with ypN_0 rectal cancer.