Introduction: Successful treatment depends on the endodontic removal of microorganisms with chemomechanical preparation. Rotary NiTi instruments offer greater flexibility and efficiency in the court, but the failure of these can occur unexpectedly by metal fatigue. Objective: To determine in vitro the first instrument that this fracture and the number of uses before observe in Mtwo® and ProTaper® rotary systems. Methodology: Experimental study in vitro preparation of comparative descriptive results. A total of 50 Rotary ProTaper® instruments and 40 Mtwo® was used to prepare Maillefer® acrylic cubes with simulated curves recorded lima which is the first of each series to be fractured and how each game uses limes stand before fracture. Results: The number of uses before breaking for ProTaper® is Mtwo® 3.9 and 9.7 is the first file in fractured ProTaper® is the F1 and Mtwo® is the 25/.06 (p 0.0001). Conclusion: The duration of Mtwo® rotary instruments is greater than ProTaper®. Relatively high incidence of fracture for F1 ProTaper® instrument system at four uses and for instrument 25/.06 of Mtwo® system was observed 10 uses, with a low incidence of deformation or change in macroscopically identifiable structure before failure occur.