Abstract: Objective To investigate mesial root canal curvatures of human mandibular first molars using cone-beam computed tomography (CBCT). Methods CBCT was performed for scanning the mandibular first molars of 1600 Chinese subjects, among whom the subjects identified to have middle mesial canals were divided into 5 age groups, namely group A (18-29 years), group B (30-39 years), group C (40-49 years), group D (50-59 years), and group E (60-80 years) for further analysis. All the CBCT images were reconstructed using Planmeca Romexis CBCT image reconstruction system, and the curvature of the mesial root was measured using the Schneider method. Results A total of 2856 CBCT images were obtained from the 1600 subjects, among whom 168 (5.88%) were found to have middle mesial canals in the mandibular first molars. The prevalence of middle mesial canals was 37.50% in group A, 17.80% in group B, 14.88% in group C, 19.64% in group D, and 10.12% in group E. The moderate of the curvature in the mesiodistal direction was significantly higher in group A than in the other groups (P<0.05); the curvature in mesiodistal direction differed significantly among the 5 groups (P<0.05), and was the greatest in group A followed by group C and group D, and was the smallest in group E. The curvature in the buccolingual direction was similar among the 5 groups. The incidence of mesiodistal and buccolingual curvature of the middle mesial canals was significantly higher in the middle 1/3 of the root canal than in the upper 1/3 and apical 1/3 of the root canal. Conclusions The incidence of middle mesial canals in the mesial root of the mandibular first molars decreases with age. The middle mesial canal system of the mandibular first molar is complex and variable, and most of the first molars have obvious curvature in the mesiodistal and buccolingual directions. CBCT can provide reference for clinical treatment to reduce the treatment failure rate.