Abstract: Objective: To investigate the value of diffusion-weighted imaging in diagnosing the metastatic lymph nodes of rectal carcinoma. Methods: 62 cases with rectal carcinoma confirmed by pathology undertake MRI common sequence and DWI scanning, the apparent diffusion of coefficients(ADC) value of 225 lymph nodes were measured. Dive the lymph nodes to metastatic lymph nodes and benign lymph nodes by the pathology. The ADC of them were compared. Results: b = 800 s/mm², the mean ADC value of metastatic lymph nodes and benign lymph nodes is(0.76 ± 0.15) × 10^-3mm²/s and(1.16 ± 0.14) × 10^-3mm²/s. There were significant differences between these ADC values(P ＜ 0.05). To distinguish metastatic lymph nodes and benign lymph with the threshold of ADC value is 1.0 × 10^-3mm²/s, the sensitivity is 94.53%, specificity is 91.62%, and accuracy is 93.45%. Conclusion: DWI posses comparatively great value in diagnosing the metastatic lymph nodes of rectal carcinoma.