Objective To evaluate the value of MRI in preoperative sub staging diagnosis of rectal cancer. Methods The selection of 33 cases of rectal cancer, preoperative MRI examination by 3.0T, do TN sub staging, and do the assessment of EMVI(Extramural Vascular Invasion), as a reference for clinical treatments, and contrast with the results of pathological staging operation, make statistic analysis. Results The total accuracy of MRI T sub staging was 81.8% (27/33), The accuracy of MRI T1～T4a staging diagnosis were 100%、93.9%、90.9%、97.0%、90.9%、97.0%、93.9%, MRI for T sub stage was approximately consistent with pathological T stage (Kappa =0.773, P<0.001). The total accuracy rate of MRI N sub staging is 69.7% (23/33), The accuracy rate of N0～N2b staging were 87.9%、90.9%、81.8%、97.0%、87.9%、93.9%, That was approximately consistent with pathological N staging (Kappa =0.619, P<0.001). The total accuracy of EMVI diagnosis was 89.3% (25/28), The accuracy of MRI E0～E2 staging diagnosis were 89.3%、85.7%、96.4%, That was approximately consistent with pathological shows (Kappa=0.747, P<0.001). Conclusion MRI has high value in the preoperative TN sub staging of Rectal cancer, and EMVI as an individual factors considered, MRI has high value in assessment of EMVI, Help to improve the refinement phase, partial evaluation factors.