Radiotherapy is the first choice for patients with nasopharyngeal carcinoma (NPC), but radiation encephalopathy affects the patients' quality of life significantly. Some research results have showed that the brain damage was associated with radiation-induced cerebral vascular injury. The study was designed to determine the cerebral blood circulation changes in NPC patients pre- and post- radiotherapy using transcranial Doppler ultrasonography (TCD), and to evaluate its value in the early diagnosis and treatment of radiation encephalopathy. The blood flow velocity of intra-carotid artery (ICA), middle cerebral artery (MCA), and basal artery (BA) were measured by TCD in 38 patients at the beginning, the end of radiotherapy, 3 months and 3-5 years after radiotherapy respectively. The flow velocity results of each cerebral artery in each examination time after radiotherapy were compared with the findings before treatment correspondingly. The velocity of both side ICA kept stable during all the follow-up time. The mean value of velocity of the left and right MCA raised obviously, from 53-54 cm/s before treatment to 59-60 cm/s at the end of radiotherapy (P< 0.05). It resumed to 54 cm/s after 3 months, and went up again to 57-58 cm/s during 3-5 years after radiotherapy, but there was no remarkable difference in statistics (P >0.05). The mean velocity of BA was 30 cm/s before irradiation, and raised to 35 cm/s at the end of treatment (P< 0.05). After 3 months, it descended to 32 cm/s (P >0.05), and attained to 34 cm/s at 3-5 years after radiotherapy (P< 0.05). During several stages of radiotherapy, the flow velocity of MCA and BA were raised obviously in NPC patients.