To assess the diagnostic efficacy of diffusion-weighted MR imaging (DWI) for evaluating inflammatory activity in patients with Crohn's disease (CD). A total of 47 CD patients underwent MR enterography (MRE) and DWI using 3 b values of 50, 400, and 800 s/mm.2 Apparent diffusion coefficients (ADCs) of inflamed and normal bowel wall were calculated. The conventional MRE findings and DWI signal intensities were qualitatively scored from 0 to 3. The correlation between Crohn disease activity index (CDAI) and both ADCs and magnetic resonance imaging scores was analyzed. Receiver-operating characteristic curve analysis was used to determine the diagnostic accuracy of CD activity. Of the 47 patients, 25 were active CD (CDAI≥150) and 22 were inactive (CDAI<150). Diffusion-weightedMR imaging and MRE+DWI scores of active CD were significantly higher than that of inactive CD (both P<0.001). Apparent diffusion coefficients in inflamed segments of active CD were lower than that of inactive CD (P<0.001).The DWI scores (r=0.74, P<0.001), ADCs (r=-0.71, P<0.001), MRE scores (r=0.54, P<0.001), and MRE+DWI scores (r=0.66, P<0.001) were all correlated with CDAI. The areas under the receiver-operating characteristics curves for ADCs, DWI scores, MRE scores, and MRE+DWI scores ranged from 0.83 to 0.98. The threshold ADC value of 1.17×10-3mm2/s allowed differentiation of active from inactive CD with 100% sensitivity and 88% specificity. Diffusion-weighted MR imaging and ADC correlated with CD activity, and had excellent diagnostic accuracy for differentiating active from inactive CD.