Cranial radiotherapy could cause several types of vasculopathies, which include atherosclerotic occlusive diseases, moyamoya disease, and aneurysm formation. To our knowledge, radiation-induced aneurysms of the internal carotid artery (ICA) are extremely rare. Here, we report a 68-year-old woman who presented with external ophthalmoplegia caused by radiotherapy after the transsphenoidal surgery for metastastic tumor of the clivus region, and the angiography demonstrated a giant aneurysm of the cavernous ICA. After the ICA ligation, the patient recovered well without brain ischemia with a 6-month-long follow-up. The present case is extremely rare with external opthalmoplegia caused by the giant cavernous ICA aneurysm, and the radiotherapy after transsphenoidal surgery might have been critical in the formation of the aneurysm.