Inserting a double lumen endotracheal tube is somewhat more difficult than a single lumen tube intubation because of its larger outer diameter and the more rigid feature. Moreover, the airway axes (oral-,pharyngeal-, and laryngotracheal axis) are not always straight under sniffing condition. Therefore, to make easy to insert a double-lumen tube and reduce the tube-related complications, we are trying to compare the individually angle-modified tube and the manufacturer-provided-angled tube.