The purpose of the present study was to compare the lifespan and deformation of three single file Nickel Titanium rotary instruments: ProTaper F2, WaveOne primary and One Shape when used in simulated curved canals. A total of fifteen defect-free instruments were divided into three equal groups according to the instruments' type. The selected instruments were used to prepare simulated canals embedded in resin blocks, which had a 45 o angle and a 6 mm radius of curvature. Both ProTaper F2 and WaveOne primary files were activated in a reciprocation motion using angles of reciprocation of 150o CW / 30 o CCW and 150 o CCW / 30 o CW respectively; whereas, the One Shape files were used in a continuous rotation. Each instrument was repeatedly used till the incidence of fracture, and the mean lifespan was determined for each group. Naked eye inspection and stereomicroscope examination were performed to examine the instruments for defects following each canal preparation, where the incidence, type, and progress of deformation for each instrument were recorded. Also, the canal preparation time was determined, and the mean time for canal preparation for each group was calculated. The ProTaper F2 and WaveOne primary instruments showed no significant difference in the mean lifespan, whereas, One Shape instrument had a significantly longer lifespan. There was no significant difference between ProTaper F2 and WaveOne primary instruments regarding the incidence of cracks, while One Shape instrument showed a significantly delayed incidence of cracks in comparison to both instruments. None of the ProTaper F2 or the WaveOne primary instruments showed visible signs of deformation prior to fracture. Even though, One Shape instruments were distorted following the first use, this distortion failed to denote the incidence of fracture. Cracks were the most frequently detected defect, and their incidence and size had a direct relation to fracture. Concerning time for canal preparation, One Shape instrument showed the shortest time for canal preparation followed by WaveOne primary instrument, Whereas ProTaper F2 instrument showed the longest time for canal preparation.