The purpose of this study is to understand how the M-WireTM alloy, a material used in many endodontic rotary files nowadays, conditions the mechanical flexibility of these instruments at body temperature. The alloys are compared in terms of their transformation characteristics by X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC), and in terms of their mechanical behaviour by numerical simulations with Finite Element (FE) models. Two different rotary instruments, a Profile GT® 20/.06 (GT) and a Profile GT® Series X™ 20/.06 (GTX), were selected due to their geometrical similarity and their different constituent alloy. GTX files are made from M-WireTM, a Ni-Ti alloy allegedly having a higher flexibility at body temperature when compared to the classical Ni-Ti alloy used in GT instruments. Both files were analysed by XRD and DSC to investigate phase transformations and the effects of working temperature on these different Ni-Ti alloys. Mechanical behaviour was assessed by means of static bending and torsional FE simulations, taking into account the non-linear superelastic behaviour of Ni-Ti materials. XRD analysis revealed that GT files present austenitic phase at body temperature (37°C), whereas GTX ones present R-phase, at temperatures under 40°C, with a potential for a much larger flexibility. For the same load and boundary conditions, FE models showed that the slight geometrical differences between GT and GTX files do not introduce great disagreement in the instruments’ mechanical response. Furthermore, it was confirmed that M-WireTM significantly increases GTX instrument’s flexibility and stress relief at the most critical zones of the rotary instruments, mainly due to the presence of R-phase at body temperature (37°C).