Anisocoria during anesthesia is very rare, but may indicate life-threatening neurologic injuries and create tremendous condition. A 23-year-old male patient was anesthetized with fentanyl and propofol. Thirty minutes after inducing anesthesia, unilateral mydriasis was detected. The duration of surgery was shortened for prompt evaluation of the pupils. At the end of the operation neither vision problem nor obvious anisocoria was detected in the recovery room. Ophthalmological and neurological consultations revealed no pathological finding and the patient was discharged. Evaluation of the size of the patient's pupils before operation would mitigate the fearful condition resulted from anisocoria.