The purpose of this study was to examine the morphological changes of different simulated canals according to the preparation procedures. With the use of clear casting resin, simulated straight and curved canals were created so that canal preparation procedures could be directly visualized and compared. Thirty clear polyester casting resin blocks which contained four simulated canals divided into three groups; Group A($0^{circ}$), Group B($15^{circ}$), and Group C($30^{circ}$). In each block, 3 canals were prepared different preparation techniques, which were conventional method, step-back method, and giromatic filing. But, one canal was not prepared as a control group. The results were as follows: 1. There was no difference on canal shape among three canal preparation methods in straight canals (Group A). 2. When conventional method and Giromatic filing were used in curved canals (Group B, C), elbow, zip and hour-glass shape were formed in apical third. 3. When conventional method and Giromatic filing were used in curved canals (Group B, C), tear-drop appearance developed at the site of the canal exit in curved canals. 4. In curved canals (Group B, C), file tend to straighten within the canal. 5. There was no difference on canal shape according to curved angle in step-back method (p > 0.1). But there was significant difference on canal shape according to curved angle in conventional method and Giromatic filing (p < 0.001). 6. Step-back method was significantly more effective than conventional method and Giromatic preparation in morphologic aspects of apical third of original canals.