Abstract： Objective To evaluate the accuracy of cone-bean CT(CBCT) on the measurement of the tooth length.Methods One hundred and sixty single root premolars extracted for orthodontic purposes were selected.The tooth length was measured with vernier caliper.The premolars were divided into 10 groups randomly,16 premolars in each group.In each group,the teeth were lined in molar area,premolar area,canine area and anterior tooth area in maxilla and mandible.Then CBCT was taken,and the tooth length was measured using the software supplied by the manufacturer in coronal and sagittal plane.Accuracy was compared between vernier caliper measurement and CBCT measurement.Results The tooth length measured by CBCT was smaller than that measured with the vernier caliper.Significant differences were found in six areas,except the maxillary and mandibular premolar areas.With the increase of mesial-distal inclination or buccolingual torque angle,the difference between tooth length measured by CBCT and that measured with vernier caliper increased.The correlation between buccolingual torque angle(∠X1) and the coronal view of CBCT radiology tooth length (Spearmen index was 7.00,P ＜ 0.001),and the correlation between mesialdistal inclination and the coronal view of CBCT radiology tooth length (Spearmen index was 0.624,P ＜0.001) were found.Conclusions The tooth length measured by CBCT was smaller than that measured with vernier caliper.