Objective To determine the root canal initial taper of mandibular second premolars of 13-23 year-old people by using Micro-CT. Methods 23 extracted, single root, human root canal of mandibular second premolars of 13-23 year-old people were selected. The teeth were embedded in resin. The resins were scanned using Micro-CT at a resolution of 10 μm. The photographs at each level of 1 mm perpendicular to the long axis of the tooth were chosen. Bucco-lingual（BL） and mesio-distal（MD） diameters of each canal were measured at cervical, middle and apical third of the canals. Finally the taper were calculated and analyzed. Results The average taper of cervical, middle, and apical third of adolescents′ root canals were 0.06, 0.08, 0.07（mesio-distal direction） and 0.32, 0.21, 0.25 （bucco -lingual direction）. The difference was not observed among three level on mesio-distal direction about the canal taper by t-test（P>0.05）, but there was difference among three level on bucco-lingual direction about the canal taper by t-test（P
