We report a case of posterior fossa arteriovenous fistula with a possible arterial aneurysm. The patient was a 15-year-old female. She was transferred to us because of a sudden onset of dyspnea, severe headaches, and loss of consciousness. The CT scan found an abnormal vascular lesion in the posterior fossa. Calcification was noted in the wall of the largest abnormal vascular structure. The angiogram showed a focal aneurysmal dilatation of the hemispheric branch of the right posterior inferior cerebellar artery, followed by an arteriovenous fistula, which drained to the right transverse sinus. Embolization was performed by implanting a detachable balloon into the proximal end of the aneurysm. These abnormal vascular structures disappeared completely on the angiogram after the embolization. CT scan one week after embolization showed increased density in these abnormal vascular structures, with no enhancement after contrast, indicating thrombosis or the formation of a blood clot in their lumen. The CT half a year after embolization showed reduction in size and a low density change in the largest vascular structure with calcification in the wall. All other abnormal vascular structures disappeared. An angiogram half a year after the embolization confirmed no recurrence of the aneurysm or arteriovenous fistula.