Aim of the study: to evaluate the effect of autoclaving on the cyclic fatigue resistance, bending properties and stiffness of the rotary endodontic NiTi instruments Revo-S (SU) (25/0.6) and RaCe (25/0.6).Methods: Eighty four instruments of Revo-S SU type and eighty four instruments of RaCe type were subjected to cyclic fatigue and bending tests. The files were classified into two main groups (each n=42) according to file. Each group was further subdivided into three subgroups (n=14) according to the number of sterilization cycles. Subgroup I(no sterilization),Subgroup П (3 sterilization cycles),Subgroup Ш (5 sterilization cycles). Custom made device of a metal testing block consists of a simulated canal with a 45˚curve and 3mm radius of curvature was used for cyclic fatigue testing. A cantilever-bending test was conducted with the use of computer controlled materials testing machine. Results: Comparing the results between unsterilized and sterilized instruments for each type of file, differences were statistically significant (P < .05) only between sterilized and unsterilized Revo files regarding the bending properties. The other instruments did not show significant differences (P > .05) in the mean NCF, bending force and stiffness as a result of sterilization cycles. Comparing the results among the different groups, Race files showed a mean NCF significantly higher than Revo-S files (P < .05) in subgroup I (un sterilized)and Revo-s files showed significant differences (P > .05) higher than RaCe files in bending force and stiffness in subgroup I(un sterilized) and subgroup Ш (5 cycles of sterilization) respectively. Conclusion: Autoclave sterilization may not affect the cyclic fatigue resistance, bending force and stiffness of endodontic rotary instruments made of conventional Ni-Ti alloy regardless the number of sterilization cycles.