Abstract: Objective To analyze fracture of Protaper nickel-titanium rotary instrument in root canal preparation and influencial factors. Methods 102 Protaper nickel-titanium rotary instruments were collected which fractured in clinical root canal preparation. Record the sizes of files, and observe the types of fracture with scanning electron microscope. The analysis was carried out in their distribution in teeth and canals, their relationship with degrees of curvature of canals, and their relationship with time. Results Most of the clinical fracture of nickel-titanium instruments was flexural fatigue which frequently occurred in molars, the apical 1/3 of roots , and canals with curvature more than 30°. Number of the broken nickel-titanium instruments in the previous two years（March 2008-Februray 2010） was 74(72.5%) while that in the later two years (March 2010-February 2012) was 28 (27.5%). Conclusion The fracture of Protaper nickel-titanium instruments is closely related to the feature of instruments themselves, the anatomy of the root canals of involved teeth, the operator′s experience, the usage of instruments and so on.