Abstract: Objective To investigate the root and canal morphology of mandibular second molars using cone-beam computed tomography. Methods Totally 372 CBCT images of mandibular second molars were obtained from 187 patients (139 men and 48 women) who were treated in Chinese PLA General Hospital from September 2016 to September 2017. The root and canal morphologies were analyzed. Results Single-, double- and triple-rooted teeth were all observed in mandibular second molars. Mesial root with two canals and distal root with single canal were commonly seen. C-shaped root canal in mandibular molars occurred in 34.95% cases, with C2 as the most common type (70.31%). Bilateral symmetry of root and canals configuration exhibited in 86.49%cases. Conclusion The variations in root and canal configuration of mandibular second molar are great, and C-shaped root canal is common. The prevalence of C-shaped root canal is higher in female than male. Symmetry of mandibular second molars exhibit in majority of patients.