Aim: The current study aimed to evaluate the impact of viscosity of castor oil on deformity and fracture of 2Shape nickel titanium instrument during root canal preparation. Methods: two hundred mandibular first molars with mesio-buccal canals having angles of curvature ranging from 20° to 40° were divided into four equal groups each of 50 according to the viscosity of lubricant: group I (Castor oil 100%), group II (Castor oil 90%,  group III (Castor oil 80%) and group IV(EDTA 17%). Then each groups divided into five subgroups (n=10). The root canals were prepared using 2Shape rotary instruments which examined before and after root canal preparation under electron microscope at different magnifications (10× and 50×) for any fracture or deformity. Collected data were then statistically compared to determine differences between the tested root canal lubricants. . Results: After using tested lubricant solutions, the Castor oil 80% showed the lowest fracture rate on 2Shape rotary instruments while EDTA 17% showed the highest rate. Castor oil 100% have the highest incidence rate of unwinding in comparison with other groups while castor oil 80% have the lowest rate of unwinding.  Conclusions: Within the limitations of the present study, Application of Castor oil 80% as intracanal lubricants cause less fracture rate on 2Shape rotary instruments compared to other canal lubrication.