Objectives:To evaluate the incidence of dentinal crack initiation after root canal preparation using four different nickeltitanium (NiTi) rotary instruments.Methods: Sixty single-rooted premolars were selected. A total of 15 teeth were left unprepared and served as a negative control; another 25 teeth were instrumented with the ProTaper universal system up to size F2 as a positive control, and the remaining 30 teeth were shaped with the following experimental groups with an apical size 25 file: ProTaper Next X2 and K3XF 25/0.4. Canals were irrigated with 12 mL of 1% sodium hypochlorite solution and rinsed with 5 mL of distilled water. All the roots were sectioned perpendicular to the long axis at 3, 6 and 9 mm from the root apex and examined under a stereomicroscope at 25X magnification to determine the presence of cracks. Data were analyzed using Fischer's exact test.Results: There was a non-significant difference in the percentage of dentinal crack after biomechanical preparation with NiTi rotary files as compared to the unprepared group except when compared with protaper universal.Interpretation and conclusion: All NiTi rotary files are capable of initiation of cracks during root canal instrumentation. The maximum percentage of cracks occur in the apical section. The percentage of dentinal crack in the apical section is least in ProTaper Next.