Introduction: The aim of this study was to investigate the effect of instrument material, taper and degree of root canal curvature, and the interaction between these factors on the cyclic fatigue of rotary nickel-titanium instruments. Materials and methods: Two types of rotary NiTi root canal instruments made from different materials were tested: ProFile GT and Profile GT Series X (both Dentsply Tulsa Dental, Tulsa, OK, USA). For each system, 60 instruments of similar size (size 30) were divided into three equal groups according to instrument taper (0.04, 0.06, or 0.08). Then each group was tested for cyclic fatigue resistance in simulated root canals exhibiting 5 mm radius of curvature, and either a 45- or 60-degree angle of curvature. The number of cycles to failure were analysed by one-way analysis of variance (ANOVA), followed by Tukey-Kramer multiple-comparison post hoc test. Then three-way analysis of variance was performed to assess the interaction between instrument material, taper and canal curvature on cyclic fatigue. Results: The results of the one-way ANOVA revealed that there was a significant effect for each of the studied variables on cyclic fatigue. The results of the three-way ANOVA revealed that the interaction between the variables significantly but unequally affected cyclic fatigue. Conclusions: The most effective variable was the degree of root canal curvature, followed by instrument taper, and finally the material used for manufacturing of the instruments. Keywords: cyclic fatigue, root canal curvature, rotary nickel titanium instruments, taper