Anaplastic thryoid carcinomas are very aggressive and lethal solid tumors; they sometimes show microscopic vascular invasions, but they rarely result in tumor thrombus in the internal jugular vein or in the other great veins of the neck. We encountered a 64-year-old woman with anaplastic thyroid carcinoma and a tumor thrombus that extended from the Lt. internal jugular vein to the innominate vein. Palliative total thyroidectomy, modified radical neck dissection and segmental resection of the Lt. internal jugular vein were done, along with removal of the intravascular tumor thrombus. We recommended postoperative radiotherapy and chemotherapy, but the patient refused additional treatments. We report here on this case for which we first experienced anaplastic thryoid carcinoma associated with tumor thrombus of the internal jugular vein.