Abstract： Objective To investigate the anatomic features of the root apexes of permanent three-rooted mandibular first molars.Methods A total of 122 permanent mandibular first molars of Han Chinese patients were collected.Twenty three-rooted and 25 two-rooted molars were scanned by micro-CT and then reconstructed three-dimensionally.The apical anatomy of the tooth models were analyzed in software Mimics 10.01.The long and short diameters of the apical constriction ( AC),the distances between AC,apical foramen(AF) and apex were measured.One-way ANOVA and LSD-t tests were used to compare the groups in relation to AC diameter and the distances between the AC,AF and apex.Results The AF of the mesiobuccal(MB) canals most frequently presented at the distal side of the apex( 10 cases in three-rooted and 6 cases in two-rooted group),and of the mesiolingual (ML) canals,most often at the lingual side (8 cases in each group).The AF of the distobuccal (DB) roots were frequently located at the distolingual (DL) side ( 10 cases),and those of the DL roots and distal canals of two-rooted molars were most often at the buccal(7 cases) and distal( 11 cases) sides,respectively.The percentage of the " classical" singular AC was 53％ (80/151).The average long(D) and short(d) diameters of the AC of the DB canals were (0.32 ±0.09) mm and (0.25 ±0.05 ) mm,respectively,significantly larger than the DL canals [ D =( 0.27 ± 0.08 ) mm,d =(0.22 ± 0.06) mm,P ＜ 0.05 ] and the M L canals [ D =(0.24 ±0.06 ) mm,d =(0.19 ±0.06) mm,P＜0.01 ).In three-rooted group,the mean distances between AC and AF,AF and apex,and AC and apex were (0.67 ± 0.32), (0.49 ± 0.28) and (1.01 ± 0.34) mm,respectively.Conclusions The AF of three-rooted mandibular molars frequently deviate from the root apex,and the AC of the DB canal is wider than those of the other canals.The mean distances between AC,AF and the apex suggest that root canal therapy should terminate at 1 to 1.5 mm short of the radiographic apex.