Introduction: The canal transportation is defined as as removing the structure of canal wall on the outer curve in the apical half of the canal due to the fact that the rotary files being used failed to restore themselves to their original linear shape during canal shaping. The success of an endodontic treatment is determined majorly by the effective biomechanical preparation and following the original anatomy of the root canal. Materials and Methodology: For the study purpose, eighty mesio-buccal root canals from human permanent mandibular first molars teeth which were extracted for various periodontal reasons, caries free and non-carious lesion free accompanied by normal anatomical form and structure were carefully selected. At once, the root canals were prepared, CBCT series of images were taken for all the teeth. Both mesiodistal and buccolingual diameters were recorded at 2mm, 3mm and 4 mm from the apex in both the diagnostic and postoperative CBCT imaging.