This is the study entitled "Comparison for oral negative pressure therapy and CPAP for OSA under sleep endoscopy assistance: A randomized controlled trial". CPAP is first line golden standard treatment for Obstructive sleep apnea (OSA). However, the overall CPAP compliance is poor. Recent study reveals that oral negative pressure therapy improves apnea severity in OSA patients with good compliance. However, no study is designed to compare oral negative pressure therapy and CPAP for OSA patients. In addition, the effect of oral negative pressure therapy for hypopharynx is unclear. Therefore, we will perform druginduced sleep endoscopy to evaluate upper airway obstruction of OSA patients before treatment. OSA patients will be assigned into oral negative pressure group or CPAP group for 2 months. Sleepiness, sleep quality, residual apnea severity and compliance will be evaluated.

Inclusion Criteria:

• mild to moderate OSA

Exclusion Criteria:

• hypopharynx obstruction
• ASA classification of physical status)>3
• allergy toDexmedetomidine
• COPD stage IV
• Asthma with exacerbation
• severe CHF
• age < 18 year-old
• claustraphobia
• any reason : can not tolerate CPAP
• second or third degree AVB