A caae is preaented of unilataral tension pneumothorax in a 5 year old patieat who underwent a cleft palate. This complication is belived to have been due to ventilation of high intratracheal pressure with high fresh gas flow (8L/min). It is suggested to use a correct sise tube, ventilation of a proper intratracheal pressure with appropriate fresh gas flow and to monitor breathing sound with precordial stethosco during E.N.T. operation.