The aim of this study was to characterize metallurgical properties and the dimensions of three types of rotary NiTi endodontic instruments and to establish a correlation between these characteristics and the flexibility of the instruments. Their chemical composition and constitution were characterized by energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD). Transformation temperatures were assessed by means of differential scanning calorimetry (DSC). Computer software was used to analyse images of the longitudinal and transverse sections for determining instrument diameter and cross-sectional area at 3mm from the tip. The flexibility of the instruments was evaluated in bending tests performed according to the ISO 3630-1 specification, in which the instruments are clamped at 3mm from their tip and bent by 45° along their longitudinal axis. The values of bending moment at 45° were correlated with instrument diameter and cross-sectional area at 3mm from the tip. The results of EDX, XRD and DSC showed that physical and chemical properties of the materials differed slightly among the files analyzed. A direct relationship was found between bending moment and the geometric characteristics of the instruments. Resistance to bending of NiTi root canal instruments depended on their geometrical shapes and metallurgical properties, but the cross-sectional configuration can be seen as an important parameter affecting this property.