Diffusion-weighted imaging (DWI) is a functional technique based on the ability to depict movement of water molecules. The magnitude of water molecule movement is expressed as apparent diffusion coefficient (ADC) value. Its usefulness in the diagnosis of malignant tumors has gained interest. The purpose of this study was to evaluate the usefulness of DWI in detecting lymph node metastases of colorectal cancer. The subjects were 46 consecutive patients (mean age 71.4 ± 8.7 years) with colorectal cancer, treated by radical surgery from 2006 to 2008. The size of metastatic lymph nodes on DWI was significantly larger than non-metastatic lymph nodes (10.3 vs. 7.6 mm). The mean ADC value was significantly lower for metastatic lymph nodes than non-metastatic lymph nodes (1.36 vs. 1.85 × 10^-3 mm²/sec). In addition, for evaluation of lymph node metastasis that reflects the primary tumor characteristics, the LN/T ratio (defined as the ratio of lymph node ADC value to the primary tumor ADC value) was calculated. It was significantly lower for metastatic lymph nodes than non-metastatic lymph nodes (1.41 vs. 1.59). Receiver operating characteristic curve analysis revealed that the best performing cutoffs were 8.5 mm for lymph node size, 1.44 × 10^-3 mm²/sec for ADC value, and 1.495 for LN/T ratio. Accuracy was significantly greater for lymph nodes with LN/T ratio (78.5%) than for lymph node size (62.0%) or ADC value (74.8%). In conclusion, preoperative DWI, especially the LN/T ratio, is recommended for evaluation of lymph node metastasis in patients with colorectal cancer.