Objective To investigate the efficiency of different angle projection radiograph in diagnosing multiple canals of mandibular first premolars. Methods Eighty- eight mandibular first premolars that needed endodontic treatment in vivo were selected. The radiograph was taken at a horizontal angles of 0, 20 - 30 degrees from mesial or distal direction of the tooth preoperative. According to the different angle projection radiographic evaluation, radiographic diagnosis of multiple canals were suspected. After the root canal orifice were detected and located with K files, the root canals were prepared using crown- down technique and obturated using cold lateral condensation technique. The configurations and numbers of root canals were identified and recorded. The postoperative radiograph was taken. According to configurations and postoperative radiograph, the root canal configuration was classified into typeⅠ - Ⅴ. Results In eighty- eight mandibular first premolars, 31 multiple canals teeth were suspected in preoperative radiographic diagnosis, 30 multiple canal teeth were detected under clinical evaluation. In case of 31 multiple canal teeth suspected by preoperative radiographic diagnosis, 3 teeth were not found multiple canals under clinical evaluation. While in case of 2 teeth of one canal suspected by preoperative radiographic diagnosis, multiple canal was detected under clinical evaluation. In 31 multiple canal teeth suspected by preoperative radiographic diagnosis, 13 teeth were detected at a horizontal angles of 0 degree and 25 teeth were detected at a horizontal angles of 20- 30 degree, 7 teeth were suspected at different angle projection. Clinical detected rate of multiple canal in mandibular first premolar were 34.09%（30/88）, 70.00% of which were type Ⅳ and Ⅴ. Conclusion Different angulation radiograph of preoperative will assist to increase clinical detection rate of multiple canals in mandibular first premolar.
