"Objectives:" The aim of this study was to compare the mechanical behaviors of endodontic Ni-Ti rotary instruments using different geometries of cross-section via finite element analysis (FEA) as well as to experimentally validate the theoretical outcomes. "Methods:" Two systems, Profile® #30; .06 taper and ProTaper® F3, were selected to analyze the stress distribution under different loading and boundary conditions via finite element model (FEM), created via CAD software, Solidworks 2004 version. On the other hand, 50 files of combinatory bending and rotational tests at different restricted points was applied on each system until instruments were experimentally fractured by an universal testing machine (Instron 5566; Canton, MA, USA). Fractured specimens were further observed by scanning electron microscope (SEM) to characterize the crack initiation and propagation facets. "Results:" The experimental data correlated very well with FEA analysis, showing that ProTaper® presented higher stress value peaks than ProFile®. This result was even more significant under the static bending loading test (p<0.05). The loop hysteresis of both Ni-Ti rotary instruments seem to be minimized to the point that even the ProTaper® group displayed “ disappear” in the dynamic rotation loading test but were statistically not significant. It is noteworthy that bending angulations ranging from 27.4° to 30° outlined the critical area affecting stress distribution of both Ni-Ti rotary instruments. SEM observations further revealed that cyclic fatigue was the predominant failure mode in the dynamic rotation test. "Conclusion:" We obtained useful information concerning instruments under cyclic tensile and compressive stresses, which can cause fatigue failure. We recognized the importance of defining the limitations of Ni-Ti instrument applications and further propose investigations into the modifications of its geometry.

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