Objective：To investigate the incidence and location of distolingual canal orifice in permanent mandibular first molars using cone-beam computed tomography（CBCT）and the relevant receiver operating characteristic（ROC）analysis. Methods：A total of 572 mandibular first molars were chosen from 300 patients undergoing CBCT scan in Department of Radiology，Stomatology Hospital of Jiangsu. The number of the distal roots and the root canals of mandibular first molars were recorded by the analysis of the CBCT data using SIMPLANT software，and the distance and angle（∠DBML and ∠DBDL）between the root orifices were measured accordingly. The best diagnostic index for the presence of the distolingual canal orifice in the permanent mandibular first molars was selected by ROC curve，and the critical value was calculated by Youden index. Then，the relationships between the angles and distances formed by the distolingual canal orifice and other root canals orifice were analyzed. Results：The prevalence of two distolingual canals in mandibular first molars was 37.2％（213/572）. The balance test of the classification of distal roots showed no statistically significant difference in gender，age and side（P > 0.05）. ROC analysis showed a high diagnostic accuracy of 0.988 of the area under the ROC curve，which suggested that the ∠DBML could be used as a prediction data to the presence of distolingual canal orifice. The critical value for the diagnosis of distolingual canal orifice of mandibular first molars by ∠DBML is 72.143°. The distolingual canal orifice can be located using the ∠DBDL and ∠DBML angle ratio（∠DBDL=0.551×∠DBML），and the MBO-DLO and MLO-DBO distance ratio（MBO-DLO=1.035 × MLO-DBO）. Conclusion：The angle of ∠DBDL and the distance of MLO-DBO in permanent mandibular first molars could be used clinically for the location of the distolingual canal orifice.