Abstract: Objective: To explore the reliability of MRI in the assessment of whether rectal cancer patients have invasive carcinoma on circumferential resection margin (CRM). Methods: Examine 7 corpses partial sectional specimens (specimen group), 10 healthy volunteers (compare group) and 49 rectal cancer patients (patient group) using MRI. Then dissect the corpse specimen and perform total mesorectal excision to the patient group. Compare the MRI result of the compare group and the specimen group, and measure the distance between mesorectal soft tissue nodules, irregular streaks shadow and mesorectal fascia of patient group MRI result. It is CRM (+) when this distance > 5mm, and it is CRM (-) if the distance < 5mm, compared with pathological findings. Result: The mesorectum fascia of both specimen group and patient group is shown on both T1WI and T2WI as a low-signal-intensity linear structure, lymph node, blood vessels, and lymphatic vessels are all shown. The MRI result is consistent with the anatomy result. Preoperative pathology results in patient group showed 22 positive CRMs, 27 negative CRMs. CRM judgment by MRI was consistent with the pathology results in 45(91.8％) of 49 patients．4 cases were misjudged: 2 patients with false positive results and 2 patients with false negative result. Conclusion: MRI is a relatively reliable method in evaluating the status of CRM.

Key words: Rectal cancer, Fascia, Rectum, MRI, Circumferential reseetion margin