Objective: To investigate the effect of dexmedetomidine on the end-tidal concentration of sevoflurane at which 50% (EC50) of the attempts to insert the Ambu laryngeal mask would be successful. Method: The patients of ASA physical statusⅠ, aged 4～10 years, undergoing elective surgery with general anesthesia were included. Anesthesia was induced with sevoflurane 8% in a mixture of nitrous oxide (N2O) 50% and oxygen 50%. After intravenous cannula was placed, N2O was discontinued and inhalation concentration of sevoflurane was modified to reach predetermined end-tidal concentration of sevoflurane. In group D, dexmedetomidine 1μg/kg was infused for 10 min. In group C, 9% sodium chloride solution was infused. 10 min later, Ambu laryngeal mask was inserted in both groups. For the first patient in group D and in group C, the end-tidal concentration of sevoflurane was designed at 1.8%. Using modified Dixon’s up-down method, the target end-tidal concentration of sevoflurane for the next patient was determined with increment size of 0.2%. Results: The values of sevoflurane required for successful insertions of Ambu AuraOnce laryngeal mask in group D and in group C were 1.44% [1.17%, 1.60%] and 1.77% [1.52%, 2.01%] respectively. Conclusion: Dexmedetomidine 1μg/kg reduces the value of EC50 of sevoflurane for insertion of Ambu AuraOnce laryngeal mask airway in children.