Objective     To determine the initial taper of single root canal of mandibular first premolar in different age groups by using micro-CT.     Methods     Fifty-eight untreated human single root canal of mandibular first premolars with complete dental root and apex were selected by random digits table. The teeth were scanned by micro-CT and divided into three groups according to age

including a 13-25 group

a 26-45 group and a 46-60 group. The root of each tooth was divided into three segments of cervical

middle and apical of the canals. Then diameters of the each section in buccolingual and mesiodistal direction were measured using Mimics 10.01 software. The physical canal tapers in buccolingual and mesiodistal direction were calculated and analyzed.     Results     In buccolingual direction

significant differences of root canal tapers in the three segments were observed between the 13-25 group and 26-45 group and between the 13-25 group and 46-60 group (P0.05)

respectively. Only root taper in the middle segment of the 26-45 group was significantly different from that of the 46-60 group (P0.05). In mesiodistal direction

significant differences of root canal tapers were observed in all three segments between the 13-25 group and 46-60 group. Significant difference was observed only in the cervical segment between the 26-45 group and 46-60 group

and only in the apical segment between the 13-25 group and 26-46 group.     Conclusion      The initial tapers of single root canal of mandibular first premolar are varied corresponding to different ages. Clinical instrument selection for root canal requires attention.