Objective : The aim of this study was to compare the cyclic fatigue resistance of two nickel-titanium (NiTi) endodontic instruments from ProTaper and GT series X files. Study Design: Cyclic fatigue test was realized with instruments from ProTaper: F1 and F3 (Denstply Maillefer, Ballaigues, Switzerland) and GT-X: 20.06 and 30.08 (Dentsply Tulsa Dental, Tulsa, Oklahoma, United States of America). A total of 320 instruments were rotated in 4 curved artificial canals with different angles and radius of curvature. The time and cycles to failure were calculated. The data was compared using a Mann-Whitney, Kruskall-Wallis, and Kolmogorov-Smirnov tests, with a significance level of p<0.05. Results: GT-X files rotated for a significantly longer period of time before separation occurred, thus GT-X files where more resistant to the cyclic fatigue compared with ProTaper. Conclusion : GT-X files have a greater resistance to cyclic fatigue, this fact can be caused by the use of the Ni-Ti alloy "M-Wire".