INTRODUCTION: This study aimed to evaluate the correlation between the fibrous area ratio and the number of cycles to failure (NCF) in terms of fatigue fracture resistance of NiTi rotary files. METHODS: ProTaper Universal (PTU) and ProTaper Gold (PTG) files of size F2 (25 mm) were tested to make cyclic fatigue fracture using a custom-made device simulating a 35° canal curvature (n = 24). Using the fractured fragments, scanning electron microscopy (SEM) was taken to analyze the fracture surfaces, focusing on fibrous areas on the cross-sections. Fibrous area ratios were calculated using ImageJ software, and NCF was determined by multiplying the time to fracture by rotational speed. Statistical analysis was performed to evaluate differences between groups and assess the correlation between fibrous area ratio and NCF (P <
.05). RESULTS: SEM analysis revealed a higher fibrous area ratio in PTU files (P <
.05). The correlation analysis indicated a weak relationship between fibrous area ratio and NCF in both groups. PTG files demonstrated significantly higher NCF compared to PTU files (P <
.05). PTG group had a significantly lower fibrous area ratio than the PTU group (P <
.05). CONCLUSION: While PTG files show improved cyclic fatigue resistance due to thermal processing, the fibrous area ratio is not a strong representing factor or predictor of NCF. Heat treatment influences the property of fracture resistance more significantly than propagation process of cyclic fatigue fracture.