Objective: To assess wear, after simulated clinical use of K3 (SybronEndo, Orange, CA, EUA) and ProTaper files (Dentsply-Maillefer, Tulsa, Oklahoma, EUA) in 46 mesiovestibular roots of human maxillary first molars, recently extracted and with mean angulation of 40°.Methods: The teeth were divided as follows: a) K3 (SybronEndo, Orange, CA, EUA) (with radial bands) and b) ProTaper (Dentsply-Maillefer, Tulsa, Oklahoma, EUA) (without radial bands). The files were assessed by scanning electronic microscopy before first use to visualize fabrication defects, and after 7 and 13 cycles of debridement in the canals. Debridement with files K3 (SybronEndo, Orange, CA, EUA) and ProTaper (Dentsply-Maillefer, Tulsa, Oklahoma, EUA) was performed in accordance with the manufacturers’ recommendations. The criteria for wear assessment were: defects (stretching, shortening/spiral inversion, presence of craters), oblique cracks and fracture. After assessment by SEM and before they were used, metal barbs on the crest of the spirals and oblique scratches due to the machining process were found in all the files.Results: In the K3 system (SybronEndo, Orange, CA, USA), after being used seven times, all the instruments showed cutting angle defects,and 25/08 and 25/10 had oblique cracks in the body of the active part leading to ductile fracture after being used for the eighth time. In the ProTaper system (Dentsply-Maillefer, Tulsa, Oklahoma, USA), after being used seven times, in addition to cutting angle defects, the files also presented cracks, and ruptile fracture occurred in F3 during use for the thirteenth time. At the end of preparation, craters were seen along the active part of the files.Conclusion: In the final analysis, it was seen that the system are equivalent with regard to the defects found, irrespective of the active part design,being proportional to the number of cycles of use, and these defects could act as points of greater fragility, favoring fracture.