Endotracheal tube (ETT) cuff herniation is a rare, and often difficult to diagnose, cause of bronchial obstruction. We present a case of outside cuff herniation of an endotracheal tube that caused pulmonary right lung atelectasis. A 29-year-old man ,a case of car accident with multiple fractures, was admitted to the emergency ward and transferred to the operating room(OR) for open reduction and internal fixation (ORIF) of all fractures .The procedures were done under general anesthesia (G/A). The past medical history of the patient did not indicate any problem. Anesthesia was induced with thiopental, atracurium and then maintained by propofol and remifentanyl infusions and 100% O2 via orally inserted ETT. The patient was positioned in left lateral decubitus position for operation. Two hours after induction of anesthesia, the oxygen saturation level dropped to 85 % and the breath sounds in the right side of the chest were weakened. The chest x-ray images showed right lung atelectasis especially in the upper lobe. The problem was disappeared after removal of the ETT. In this case, we observed that an ETT cuff herniation can be a cause of airway obstruction. If there is a decreased unilateral breath sounds, we recommend replacement or repositioning of ETT.