To investigate the significance of diffusion tensor imaging (DTI) for patients with nasopharyngeal carcinoma (NPC) and trigeminal nerve invasion. Fifty-two patients with NPC and unilateral infringement and 30 healthy controls were recruited for our study. Routine magnetic resonance imaging (MRI) and DTI were performed for all participants. Within-group and between-group comparisons of DTI metrics, including fractional anisotropy (FA) and the apparent diffusion coefficient (ADC) of the third (V3) branch of the bilateral trigeminal nerves of all participants, were carried out. The FA and ADC values on the affected sides of patients revealed a significant decrease and increase, respectively, when compared with those on the unaffected sides of patients and the healthy controls (P=0.000 for all), whereas there were no significant differences in DTI metrics between both sides of healthy controls or between the unaffected sides of patients and the healthy controls (P=0.930, 0.580, 0.095, and 0.360, respectively). The decreasing FA rate on the affected sides of patients correlated negatively with the increasing ADC rate (r=-0.675, P=0.000). DTI can quantitatively evaluate microstructural abnormalities of the V3 branch of the trigeminal nerve in patients with NPC, which is important for the early detection of trigeminal nerve invasion to achieve a precise T classification, assess prognosis, and guide treatment.