This paper introduces the experimental principle and method on undulatory dorsal fin propulsion of gymnarchus niloticus, a large aggressive fish whose swimming mode is referred to as amiiform mode. The fish is propelled by undulations of a long-based dorsal fin while the body axis is in many cases held straight in swimming. In order to investigate the undulatory dorsal fin propulsion and its potential for providing alternative approaches for future underwater vehicle design, the kinematics of steady forward swimming of gymnarchus niloticus was described and the geometric features of the body and locomotive characteristic and parameters of the flexible dorsal fin were discussed. Furthermore, a simplified kinematic model on locomotion of the undulatory long dorsal fin propulsion was established according to the observation and experimental data. By applying the large-amplitude elongated-body theory, the estimation shows that the hydromechanical efficiency of the undulatory long dorsal fin system is more than 83.12%. It is suggested that the undulatory long dorsal fin propulsion is an adaptation to swimming with high hydromechanical efficiency.