Twenty new files were selected from RB（25/0.08），MP（25/0.06），HCM（25/0.06），OP（25/0.06）and PB（25/0.06）. Each group was further divided into two subgroups with 10 files in each subgroup： RB group， HCM group， MP group， OP group and PB group under 60 ° bending simulated root canal and RB group， HCM group， MP group， OP group and PB group under 90 ° bending simulated root canal.The root canal motor was set according to the speed and torque recommended by the manufacturer.The flexural fatigue resistance of the nickel-titanium files was tested in the curved simulant canals which were made of stainless steel with an inner diameter of 1.5 mm， 60° and 90°angles of curvatures and a radius of curvature of 5 mm.The time from rotation to fatigue fracture （TTF） of each nickel-titanium file was recorded with a timer， and the length of the fracture segment （FL） was measured and recorded with a vernier caliper.After the experiment， the broken nickel-titanium files were collected， and scanning electron microscopic（SEM） evaluation was performed at the fracture sites to investigate the morphology of fracture.

In 60 ° curved simulated root canals， the TTF of nickel-titanium file in RB group was the longest （P<0.05）， followed by HCM group and MP group （P< 0.05）， and OP group and PB group were the longest（P<0.05）；there was no statistically significant difference in TTF between HCM and MP group（P>0.05）， and there was no statistically significant difference in TTF between OP group and PB group （P>0.05）.In 90 ° curved simulated root canals， the TTF of nickel-titanium file in RB group was the longest（P<0.05）， followed by MP group （P<0.05），and HCM group， PB group and OP group were the lowest（P<0.05）；there were no significant differences in TTF among HCM group， PB group and OP group （P>0.05）.The TTF of nickel-titanium files in each subgroup in simulated root canal bending at 90° were significantly lower than those corresponding subgroups at 60° （P<0.05）.There were no statistically significant differences in FL among all groups （P>0.05）.The cross sections of nickel-titanium files in each group showed typical characteristics of cyclic fatigue including fatigue striations and fibrous dimples under SEM.

RB nickel-titanium file shows statistically higher cyclic fatigue resistance in artificial canals with 60° and 90° than the other nickel-titanium files.