The purpose of this study was to analyze in vitro the dynamic fracture of some endodontic hand instruments. The K file and H file were tested as reference traditional files; the K-Flex, Flexofile, Unifile, and Helifile were tested as newly developed hybrid instruments. The files were mounted in a lathe and rotated with cyclical axial motion in the lumen of a curved tempered steel groove until failure occurred. The results showed that the life span of instruments, the distance between the instrument tip and the rupture point, and the resistance to fracture were dependent on the size and design of the instruments. Moreover, scanning electron microscopic photographs revealed two types of breakage patterns. The K file, H file, Unifile, and Helifile showed a distinct fracture starting point with crack striations and ductile fractures. The K-Flex and Flexofile showed only plastic deformations and axial fissures.