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Background and Objective: Numerous anatomical abnormalities or pathological conditions can cause upper airway obstruction in obstructive sleep apnea syndrome (OSAS). Muller’s maneuver (MM) is one of diagnostic modalities investigating the obstruction site in patients with OSAS. This study aimed to investigate the obstruction sites of patients with OSAS based on MM. Materials and Methods: This was a case-series study. A total of 145 patients were enrolled in this study. The awake MM (a flexible fiberoptic endoscopy of the upper airway while patients perform forced inspiration against a closed oral and nasal airway) was performed by a single surgeon with the patient in a supine position. Endoscopic findings were classified using the modified velum, oropharyngeal lateral walls, tongue base, and epiglottis (VOTE) classification criteria. Results: Mean ± standard deviation age of patients was 41.5 ± 10.1 years old. Mean respiratory disturbance index was 29.7 ± 24.3/hours. The most common site of obstruction in all patients was velum. About 72% of the patients had more than 75% obstruction in the velum area while most patients had < 50% obstruction in oropharyngeal lateral walls (41.4%) and tongue base (55.2%). 69% of the patients had no obstruction in epiglottis according to the modified VOTE classification. Conclusion: Simple awake diagnostic test before surgery would help physicians to identify obstruction sites of OSAS patients.