Background Patients with neurofibromatosis type 1 (NF1) have various vascular diseases due to the vascular fragility, but no reports of case of giant thrombotic aneurysm was found. We treated a rare case of giant thrombotic aneurysm of the internal carotid artery (ICA) in a patient with NF1. Case Presentation A 60-year-old man had suffered deteriorating visual loss and homonymous hemianopia. Contrast-enhanced computed tomography showed a giant thrombosed aneurysm on the anterior wall of the ICA located in the optic chiasma. We planned and completed the external carotid artery-middle cerebral artery high-flow bypass using radial artery graft. The visual fields test was performed 14 days after surgery. Homonymous hemianopia persisted but no exacerbation of visual field impairment was observed. No complications were found at 14 days after surgery and the postoperative course was uneventful. Conclusion We consider that external carotid artery-middle cerebral artery bypass surgery using radial artery grafts is a safe and effective treatment method for giant thrombotic aneurysm associated with NF1.