The anterior talofibular ligament, one of the most vulnerable structure against inversion stress, takes origin from the anterior border of the distal fibula and inserts on the neck of the talus. Previous studies have reported that anterior talofibular ligament is nearly parallel to the long axis of the tibia at the plantar flexion and to the long axis of the talus at the weightbearing position. So, the anterior talofibular ligament stabilizes the joint on inversion stress at the maximal plantar flexion position of the ankle. The purpose of this study was to clarify the anatomic and biomechanical properties of the anterior talofibular ligament. In seven fresh below-knee amputated ankles(without fracture, infection or other ligamentous injuries, etc), the authosr measured the change of the length and with of the anterior talofibular ligament at varying position of the ankle, and calculated yield point, break point, Young's modulus and energy absorbing capacity of anterior talofibular ligament at the plantar flexion 30 degree of ankle, possibly the most taut or vertical position of the anterior talofibular ligament. The Instron-4202 wss used for this experiment study. Authors studied the ankle by repeated stress at the speed to 10mm/min within the physiological range of elasticity, and then 50cm/min, the condition of acute ankle joint injury. We also observed whether the ligament was ruptured at the attachment site or substance portion.