To investigate the reason of extraction, length, number and transverse morphology of the root and root canals of first permanent mandibular molars. 100 extracted first permanent mandibular molars were collected. The length of mesial and distal root of the first permanent mandibular molars was 13.00 mm and 13.51 mm separately. 75% double roots and 25% three roots were observed under the stereomicroscope. Difference of root number and shape of transverse morphology in mesial root were more obvious than those in distal roots of the same first mandibular molars in this study.

To investigate the reason of extraction,length,number and transverse morphology of the root and root canals of first permanent mandibular molars.100 extracted first permanent mandibular molars were collected.The length of mesial and distal root of the first permanent mandibular molars was 13.00 mm and 13.51 mm separately.75% double roots and 25% three roots were observed under the stereomicroscope.Difference of root number and shape of transverse morphology in mesial root were more obvious than those in distal roots of the same first mandibular molars in this study.