Among the hazards in anesthesiologic field, the radiation exposure is not uncommon and is mainly from the exposure to imaging equipment. The presented case is a very extraordinary one of radiation exposure from the patient taking radioactive iodine (I-131). Recently we experienced a radiation exposure during the emergency surgery of a thyroid cancer patient who was administered I-131 before surgery. The oral administration of I-131 is recognized as a standard medical treatment for thyroid carcinoma. But the patient treated with I-131 can be a potential radiation source of external exposure to surroundings and the radiation was actually near the recommended limit of yearly exposure in spite of short surgery time. Hereby we report the case and contemplate the peri-operative risk of radiation exposure.