An 81-year-old male presented with right oculomotor nerve paresis and left hemiparesis caused by a mass effect of a large superior cerebellar artery aneurysm. Endovascular treatment was performed using Guglielmi detachable coils. The patient subsequently suffered aggravation of the mass effect 3 weeks after the embolization. Bilateral vertebral artery occlusion was performed, which decreased the cerebral edema surrounding the aneurysm, but his neurological symptoms did not improve. Parent artery occlusion is recommended as the first choice of treatment for an unclippable large or giant aneurysm causing a mass effect on the brainstem.