Abstract: Objective To investigate the relationship between the expression of Ki67 and the changes in SUVmax of nasopharyngeal carcinoma before and after radiotherapy. Methods (1) Seventeen nude mice transplanted nasopharyngeal carcinoma xenografts were randomly divided into the control group, the 6Gy irradiated group and the 12 Gy irradiated group.The radiation groups underwent 18F-FDG PET-CT examination before and after radiotherapy and the expressions of Ki67 were detected by immunohistochemistry after radiation.Then the changes of the Ki67 expression and T/NT ratios in xenografts were analyzed.(2) Twelve untreated patients with nasopharyngeal carcinoma underwent 18F-FDG PET-CT examination before radiotherapy, during radiation when 50 Gy was given and 1 week after radiation, respectively.Then the SUVmax of nasopharyngeal carcinoma was measured.The expression of Ki67 in patients' paraffin blocks before radiotherapy were detected by immunohistochemical method, and then the correlation between the Ki67 expression and the changes of SUV max during radiation were analyzed. Results(1) The expression of Ki67 antigen in xenografts reduced significantly with increasing radiation dose, and the expressions of Ki67 among different groups were dramatically different (F=57.8, P<0.01).(2) The T/NT ratios among the different groups were dramatically different (F=14.0，P＜0.01).(3) The SUVmax of lesions in patients highly expressed Ki67 was significantly lower than that in patients with lower Ki67 expression when received 50Gy radiation (t=2.235,P<0.05).But there were no statistical difference before radiotherapy (t=0.176,P>0.05) or after 12Gy radiation(t=1.106,P>0.05). Conclusion The Ki67 expression is an important factor contributing to the FDG uptake of nasopharyngeal carcinoma.And the expression of Ki67 before radiotherapy may have an important impact on the reduction of SUVmax of nasopharyngeal carcinoma during radiation therapy.