INTRODUCTION:The purpose of this study was to compare the efficacy of the cutting ability of two different instruments, concerning changes in the dentin thickness removed and root canal volume, by using multislice computed tomography scanning. METHODS:Thirty single-rooted mandibular premolars were divided into two equal groups according to the preparation system used: the twisted file (TF) and ProTaper (Dentsply Maillefer, Ballaigues, Swizerland). Dentin thickness along the whole length of the root canal and canal volume were measured before and after instrumentation by using multislice computed tomography scanning and image analysis software. RESULTS:ProTaper removed significantly more dentin from the mesiodistal and buccolingual directions of the root canal than the TF (P < .05). No significant difference was recorded for the changes in root canal volume between the two systems (P > .05). CONCLUSIONS:The TF system was found to cut dentin efficiently with more uniform cutting than ProTaper system.