Abstract： Objective To investigate the imaging characteristic of DW1,tumor cell apoptosis and proliferation of nasopharyngeal carcinoma (NPC) xenograft transfected hNIS gene after 131Ⅰ treatment.Methods The NPC xenograft models were established with CNE-2-hNIS cells(experimental group) and CNE-2 cells(control group) respectively.After i.p.injections 131Ⅰ in the experimental group and control group,the changes of xenograft tumor volume and ADC value were observed in 3,6,12,18,24 days by MR scan.The correlations of changes of ADC with pathological TUNEL,Caspase-3 immunohistochemistry of apoptosis,and Ki-67 proliferation detection were further investigated.Independent samples t-test were compared between the two groups and Pearson linear correlation analysis was used.Results The tumor volume of the experimental group was significantly reduced compared with that of the control group after 131 Ⅰ treatment (t values:8.27-19.46,P ＜0.05 ).DW-MRI showed that in the 3th day after 131Ⅰ treatment,ADC values increased in the experimental group,and ADC values reached the peak in the 6th and 12th day after 131Ⅰ treatment,then ADC values were decreased. ADC values after treatment was positively correlated with apoptotic index ( r =0.72,P ＜ 0.05 ) and caspase-3 positive rate ( r =0.65,P ＜ 0.05) and there was a negative correlation with Ki-67 proliferation index ( r =- 0.71,P ＜ 0.05 ).Conclusion ADC values can reflect growth inhibition of NPC xenograft transfected hNIS gene after 131 I treatment.It is possible that DWMRI techniques can be used in early non-invasively monitor the therapy effect of 131Ⅰ treatment of NPC xenografts transfected hNIS gene.