The trigeminovagal reflex manifests as a sudden onset of bradycardia, hypotension, and cardiac arrest in response to the stimulation of the trigeminal nerve. The incidence of trigeminovagal reflex in maxillofacial surgical procedures is approximately 1.6%. We report a case of asystole in a pediatric patient in whom a dental mouth gag triggered the trigeminovagal reflex during oral surgery. The patient was a 5-year-old boy who was scheduled to undergo extraction of the maxillary supernumerary teeth. After tracheal intubation, anesthesia was maintained with sevoflurane and remifentanil. At the beginning of the surgery, his mouth was opened with a dental mouth gag, and electrocardigram showed asystole for 20 second. Thereafter, his heart rate spontaneously returned to basal value within 60 second. Since sufficient mouth opening was required to conduct the surgery, his mouth was opened again with the gag. When the interincisal distance exceeded about 40 mm, his heart rate suddenly decreased, but spontaneously returned to baseline within 60 second. The subsequent anesthetic course was uneventful.