Objective To evaluate the diagnostic performance of preoperative multi b values of DWI and ADC for the prediction of extrathyroidal extension (ETE) of papillary thyroid carcinoma (PTC).Methods Between January 2013 and February 2017,MR images including dynamic contrast-enhanced MR imaging (DCE-MRI) and DWI images of 81 patients diagnosed as papillary thyroid carcinoma in the Affiliated Renmin Hospital of Jiangsu University were retrospectively analyzed.ADC values were measured on solid regions of tumors.The differences of ADC were compared between tumors with total ETE(minimal ETE,extensive ETE) and without ETE by independent-samples t test.Results When b=500 s/mm2,ADC values of PTCs with ETE[(1.27±0.17)× 10-3mm2/s]were significantly lower than those from PTCs without ETE [(2.12±0.72)× 10-3mm2/s,(t=9.126,P=0.000)].ADC values of PTCs with extensive ETE[(1.23±0.17)× 10-3mm2/s] were significantly lower than those from PTCs with minimal ETE[(1.29±0.16)× 10-3mm2/s,(t=1.467,P=0.147)].When b=500 s/mm2,the cutoff value of ADC to discriminate PTCs with and without ETE was 1.530×10-3 mm2/s with a sensitivity of 69.0％,specificity of 93.7％,positive predictive value of 77.6％,negative predictive of 77.5％ and ROC curve area of 0.887.Conclusion ADC values of the solid tumor tissue of PTC with ETE are significantly lower than those of PTC without ETE.DWI may be helpful in the determination of thyroid papillary carcinoma ETE.