A 30-year-old male presented with subarachnoid hemorrhage, due to a rare peripheral posterior inferior cerebellar artery aneurysm located in the spinal canal. Angiography and magnetic resonance imaging disclosed the aneurysm of 15 mm diameter at the C-1 level. After aneurysmal neck clipping and ventriculoperitoneal shunt implacement, he was discharged without neurological deficit. Histological examination revealed no layer formation, only hemorrhage and organization in the aneurysmal wall. This case may be an example of giant aneurysm formation caused by repeated bleeding and organization.