The size selection of Laryngeal Mask Airway classic is usually followed the manufacturer's recommendation based on the patient's ideal weight. But sometimes the patient is tall and slim, the actual weight is much less than the ideal weight. This phenomenon might affect the success rate of insertion. Previous studies had demonstrated that the selection of Laryngeal Mask Airway size based on ideal weight could improve the success rate of insertion; therefore, this study was designed to prove whether this conclusion also applies to the tall and thin patients.