The purpose of this study was to assess the topographicalcharacteristics and metallographic structure of RaCe Ni-Ti rotaryinstruments after mechanical preparation and sterilization cycles. A totalof fifty RaCe rotary instruments were used; 5 intact files represented thecontrol Group I. Groups II, III and IV were divided equally into fifteenfiles each. Group II was subdivided into 3 subgroups M1, M2 and M3representing files used to prepare 1, 4 and 8 root canals respectively.Group III was subdivided into 3 subgroups S1, S2 and S3 representingfiles subjected to 1, 4 and 8 sterilization cycles. Group IV was subdividedinto 3 subgroups MS1, MS2 and MS3 representing files used to prepare 1,4 and 8 root canals followed by a sterilization cycle after each use.Topographical characteristics, surface roughness and microhardness ofrepresentative files were determined using environmental scanningelectron microscope (ESEM), a stereomicroscope and Vicker’s hardnesstester respectively. The crystallographic structure changes weredetermined using x-ray diffractometer (XRD). XRD proved that using thefiles in mechanical preparation of root canals up to 8 times and exposureto 8 sterilization cycles did not change the crystalline phase of RaCe NiTirotary instruments. However, the repeated mechanical preparationproduced various surface defects and frequent incidence of fracture