NiTi wires have been widely used to produce dental files for endodontic application purpose using rotary machine. In this aspect, the mechanical resistance to Rotary Bending Fatigue – RBF is an important parameter. Previous works have shown that the resistance to RBF depends upon the rotational speed, strain amplitude and carbon content of the alloy. It was also shown that the presence of wire surface damages associated to mechanical processing stages play a very important role for RBF resistance, quite often hiding the effect of carbon content. In this work, the effect of carbon content and microstructure on the resistance to RBF is better evaluated by preparing special wires with surface defects free, varying carbon content and alloy composition.