Abstract： Objective To investigate the value of diffusion-weighted magnetic resonance imaging (DWMRI) in gross tumor volume definition and localregional recurrence/distant metastasis prediction in nasopharyngeal carcinoma ( NPC). Methods From Jun. 2008 to Feb. 2011, each of 36 patients with or without Iocalregional recurrence and/or distant metastasis treated by intensity-modulated radiotherapy in our hospital was matched in this study. The difference of apparent diffusion coefficient ( ADC) values between these two groups was compared by t-test method. The conventional MRI and DWMRI before treatment were centrally reviewed to evaluated whether DWMRI would benefit to the definition of gross tumor volume.Results The mean ADC value were (0. 753 ±0. 091) x 10 -3 mm2/s and (0. 793 ±0. 094) x10-3 mm2/s in treatment failure group and control group, respectively ( t = - 1. 79 ,P = 0. 078) . The extension of gross tumor volume were much more outstanding in DWMRI when compared to conventional MRI. DWMRI predicted localregional recurrence more accuracy than conventional MRI in one patient . Conclusions DWMRI is help to definition of gross tumor volume. The ADC value before radiotherapy can not predict the localregional recurrence and/or distant metastasis for nasopharyngeal carcinoma.