This study aimed to compare the cyclic fatigue fracture resistance of two rotary Ni Ti instrument systems. The instruments compared were a new rotary system Protaper Next rotary endodontic file system and One Shape rotary file system. The cyclic fatigue testing was conducted with the instrument rotating freely at an angle of curvature 60⁰ with maximum radius of curvature at 5mm from the tip. Ten endodontic rotary files were selected in each of the two groups to be tested. The files were rotated at 375 rpm using the Tri auto mini endomotor set at 2.5 Torque, until fracture occurred. The time until fracture was recorded in seconds by using a stopwatch, and the number of rotations to fracture was then calculated and results were then statistically analyzed. The results showed One Shape (Group 2) performed significantly better than Protaper Next rotary endodontic file system (Group1) in cyclic fatigue testing.