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BACKGROUND: Root canal files fracture is a kind of common oral clinical complications. Therefore, it is of clinical importance to study the cyclic fatigue of different types of stainless steel root canal files.OBJECTIVE: To explore the effect of the bending angle and cross-sectional area of root canal instruments on cyclic fatigue of stainless steel files.METHODS: Thirty 25# stainless steel K files (25 mm in length) were selected, the same to 30# and 35#. And these three kinds of files were respectively randomized into three groups (n=10 per subgroup). In the nine subgroups, the files were placed into self-made root canals at the bending angle of 45°, 60°, 90°, and driven by a 16:1 reducer (350 r/min).Fracture time in each subgroup was recorded and converted into the number of rotations. The microstructure of the fracture end of the files was observed under scanning electron microscope. Fractured files were collected and the crack tip length was measured by an electronic caliper.RESULTS AND CONCLUSION: At the same bending angle, the number of rotations resulting in file fracture was significantly reduced with the increasing of the cross-sectional area of the files (P < 0.05). When the type and cross-sectional area were same, the number of rotations resulting in file fracture was significantly reduced with the increasing of the bending angle of the root canals (P < 0.05), indicating a reduction in the cyclic fatigue performance of the files. Toughness fracture occurred in all the files, and circular or oval roughness nests with different sizes and microcavities formed on the fracture surface. In addition, brittle surfaces could be detective between the roughness nests. The diameter of roughness nests was gradually increased with the increasing of the bending angle of the root canals. For 30# and 35# files, the number of brittle surfaces was gradually increased with the increasing of the bending angle of the root canals. For 25# files, the crack tip length was gradually decreased with the increasing of the bending angle of the root canals, but there was no significant difference (P > 0.05). For 30# and 35# files, the crack tip length was significantly shortened with the increasing of the bending angle of the root canals (P < 0.05). But there were no significant changes in the 35#-60° and 35#-90° groups (P=0.095). At the same bending angle, there were no regular changes in the crack tip length in the different types of files. In conclusion, with the increasing of the cross-sectional area, the cyclic fatigue performance of the files with the same length and taper is reduced and the files become easy to be fractured; with the increasing of the bending angle of the root canals, the files with the same types are apt to be fractured near the root tip, indicating the shorter length of the crack tip indicates the more difficulty in the removal of the broken files.