Optic nerve injury or blindness after head trauma has well been known since the days of Hippocrates, and has still remained one of the serious posttaumatic sequelae. Statistically about 2% of head-injured persons suffer from the visual loss and are impelled to have limited social activites3). As the victims of traffic accidents are increasing every year, the studies about this problem are urgent. Clinical manifestations of the injury of the optic nerve have been well known. The reports of many institutions have shown almost the same results. The summaries are as follows3): There is a constant mode of trauma to make an injury. It occurs frequently in motorcycle accidents. Sometimes it occurs even in trivial accidents, where impact force is not always so large. The blow is usually given to the front of the head. Symptomatologically visual loss occurs immediately after the trauma and is limited usually to the eye on the ipsilateral side to the impact and spares the eye on the contralateral side. Diagnosis is guided from the loss of direct light reflex of the affected side. The fracture of the optic canal is often visualized in X-ray studies. Surgical intervention is not fully effective. From the pathological view point, there are so many therories as to the causative factors of the injury of the optic nerve. The fracture of the optic canal, the bleeding of the sheath of the optic nerve, and the tearing of nerve fibers are prominent among them. Fracture theory seems most attractive because the fracture of the optic canal is found in 60-90% cases in X-ray studies. So far, these clinical and pathological data have strongly suggested that mechanical factors play an important role to cause the lesion. However, there has never been a fundamental engineering study. Therefore in this paper, mechnical studies were performed to investigate the mechanism and to clarify the threshold of the impact force of the fracture of the optic canal, which is thought to be closely relevent to the ipsilateral optic nerve injury. The studies were made up of series of experiments.