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.

One hundred and eighteen patients adult patients，actual body weight much less than ideal body weight will be assigned randomly to two groups (Actual weight Group or Ideal weight Group). Size selection was guided by the recommendations of manufacturer, patients will be based on the actual body weight in Actual weight Group while ideal body weight in Ideal weight Group.

The success rate of first attempt，number of adjustments, insertion time, ease of insertion, overall success rate, oropharyngeal leak pressure and fiberoptic view will be recorded and compared. The occurrences of respiratory complications such as sore throat, hoarseness, and dysphonia will be observed and recorded.

Inclusion Criteria:

1. Adult Patients undergo elective surgery of American Society of Anesthesiologists (ASA) physical status 1-2, aged from 18 to 60;
2. Patients' actual weight much less than ideal weight.

Exclusion Criteria:

1. Patients with gastroesophageal reflux disease;
2. Patients with predicted difficult airway;
3. Patients with preexisting airway diseases or significant upper airway infection.