The incidence of traumatic intracranial aneurysms are rare. In review of literature, only six cases of traumatic aneurysms in posterior fossa are reported. The patient, 15-year-old girl, was admitted on Dec. 18th, 1979, presented with unconsciousness after a traffic accident. She was developed impaired consciousness and left hemiparesis with right hemotympanum and nasal bleeding. Skull film showed an occipital linear fracture and a laminal fracture of the atlas. CT scan demonstrated traumatic subarachnoid, intraventricular hemorrhage and pneumocephalus. After accident, she regained consciousness with conservative therapy, but she deteriorated on the 10th hospitalized day. On the 17th day, lumbar puncture and CT scan revealed recurrent subarachnoid and intraventricular hemorrhage. The saccular aneurysm on the lateral medullary segment of the right posterior inferior cerebellar artery was obtained by cerebral angiogram on the 35th day. The configuration and size of the aneurysm was easily demonstrated by CT scans and cerebral angigrams. On the 43rd day, the clipping of the common trunk of the right posterior inferior cerebellar artery was employed. Her postoperative course was uneventful and she was discharged on the 97th day in good condition. This aneurysm was suggested to be induced by severe head injury with clinical ad operative findings. With the introduction of CT scan, CT scan is more available diagnostic modality than cerebral angiography in the follow up study with severe head injuries. And in the absence of pathological CT scan findings, suspicion of traumatic cerebrovascular impairments such as aneurysm should be paid. As the conservative management of traumatic intracranial aneurysms carries high mortality rate up to 30-50%, the prompt diagnosis and direct surgical intervention are recommended.