Dolphins swim by vertical oscillation of their tail fins which have been considered to play an important role for generating thrust. The propulsive performance of the tail fin relates with the structure and the mechanical properties of the tail fin. Our objectives are to investigate the structure and the bending properties of the central part of the tail fin of a dolphin. A sample of the tail fin was anatomized to investigate the vertebrae structure and tendons insertions, and the bending tests were carried out on the central part of the tail fin at the state with tensions on the tendons. Our results demonstrated that there were three kinds of tendons in the tail fin, that is, the epaxial tendon I, the epaxial tendon II and the hypaxial tendon. The epaxial tendons Is inserted into each tail vertebra, the epaxial tendon IIs inserted into five tip tail vertebrae, and the hypaxial tendons inserted on the chevron bones and tail vertebrae. In the bending test, it was found the tension which was acted on the epaxial tendon IIs produced larger bending deformation and made the central part of the tail fin stiffer than the same tension which was acted on the epaxial tendon Is or hypaxial tendons.