of studyis to compare the cyclic fatigue resistance of GTX andProTaper rotary files. Materials and Methods:Forty instruments were used in this study, divided into two groups, and each group is divided into 2 subgroups each containing Ten instruments so (n=10) , Group 1 contains files with apical diameter of 0.20 mm GTX 20/.06, and ProTaper F1 while Group 2 contains those having apical diameter of 0.4mm GTX 40/.06, and ProTaper F4 .A specially designed custom-made cyclicfatigue testing devicewas usedinthisstudy,withasimulatedrootcanalwitha45°angleof curvature and 5 mmradius of curvature wasconstructed. The instruments were allowed to rotate freely inside the artificial canal till fracture according to the manufacturer`s recommendations, and the time to fracture were recorded using 1/100 stop watch. The longer time to fracture indicate more resistance to cyclic fatigue. Results:The results obtained from the cyclic fatigue test showed that GTX files are more resistant to cyclic fatigue when compared with the ProTaper files (P < 0.005). Conclusions:Files made from the thermomechanically treated M-Wire alloy have higher resistance to cyclic fatigue than those made from the conventional Ni-Ti alloy.