Aneurysms of the petrous internal carotid artery are rare, and surgical treatment of the aneurysm in the petrous bone carries high risk with procedure. We report a case of large petrous internal carotid artery aneurysm associated with otitis media. A 58-year-old female was admitted to our institution with left hearing loss and facial palsy. She had 4-year history of left exudative otitis media. Imaging studies demonstrated a 17 mm left petrous internal carotid artery aneurysm, destroying osseous partition between carotid canal and Eustachian tube, and protruding into left middle cranial fossa. Endosaccular coil embolization with balloon assist technique was performed successfully without neurological deficit. Follow up magnetic resonance angiogram demonstrated complete obliteration of the aneurysm without parent artery occlusion. Traumatic, mycotic, radiation, and congenital origin have been implicated for petrous internal carotid artery aneurysm. In a review of the literature, 11 cases of mycotic aneurysms in this location were reported and all of them were complicated with chronic otitis media. Because of its close proximity of middle ear and Eustachian tube, adventitial infection of the artery weakens the arterial wall, and gives rise to formation of aneurysm. We conclude that our case is of mycotic origin, and endosaccular coil embolization is effective and less invasive treatment.