Abstract: Objective To investigate the feasibility of apparent diffusion coefficient(ADC) values of MR diffusion weighted imaging in predicting the sensitivity of nasopharyngeal carcinoma to radiotherapy. Methods MR diffusion weighted imaging was performed in 97 cases of nasopharyngeal carcinoma patients before radiotherapy, and we measured the ADC values of tumor foci. According to the radiotherapy effect and sensitivity, the patients were divided into radiation sensitive group(74 cases) and radiation resistance group(23 cases), to analyze the relationship among radiotherapy sensitivity, ADC values, pathological types and T stages. Results In radiation sensitive group, the ADC value was (0.792±0.121)×10^-3mm²/s, 23 cases were non-keratinizing differentiation carcinoma, 51 cases were non-keratinizing undifferentiated carcinoma, ten cases were T1 stage, 33 cases were T2 stage, 22 cases were T3 stage and 9 cases were T4 stage. In radiation resistance group, the ADC value was (0.730±0.104)×10^-3mm²/s, eight cases were nonkeratinizing differentiation carcinoma, 15 cases were non-keratinizing undifferentiated carcinoma, three cases were T1 stage, four cases were T2 stage, seven cases were T3 stage and nine cases were T4 stage. There were significant differences in ADC values (T= 2.199, P=0.030) and T stages (χ ²=10.287, P=0.016) but no significant differences in pathological types (χ ²= 0.111, P=0.740) between two groups. There was a significant difference in ADC values among all T stages (F=8.597, P=0.000). With the increasing of T stage, the ADC value was decreased. The area under the ROC curve(Az) value of ADC value and T stage to predict the sensitivity to radiotherapy on nasopharyngeal carcinoma patients were 0.657 and 0.661. When ADC value 0.737×10-3mm2/s was taken as the diagnostic threshold, the diagnostic value was the biggest, the sensitivity was 68.9% and the specificity was 69.6%. Conclusion ADC value could predict the sensitivity of nasopharyngeal carcinoma patients to the radiotherapy. Its value is close to T stage and the best diagnostic threshold was 0.737×10^-3mm²/s.