The torsion property and rotary fatigue fracture behavior of SmartÒ K file and R reamer made of nickel titanium alloy have been studied by means of Shimadzu Autograph DDS-10T universal tester and man-made cyclic system with curved copper tubes as simulated root canal. The instrument surface and the cyclic fatigue fracture surface have been investigated by optical microscope and scanning electron microscope. Comparing with ANSI/ADA specification no.28, maximum torsion values of both SmartÒ K and R endodontic instruments are satisfied in practice. Tensile stresses on the outer surface of the endodontic instruments play a dominant role in fracture initiation during cyclic fatigue fracture. The cyclic number corresponding to cyclic fatigue fracture decreases with the increase of the instrument size number/ curvature. SmartÒ K- and R endodontic instruments exhibit good ductility characteristics, therefore superior resistance to fracture.