Obstructive sleep apnea and hypopnea syndrome (OSAHS) is characterized by obstruction of upper airway and respiratory disturbance, excessive daytime sleepiness and tiredness.The possible causes are obesity, hypertension, and upper airway malformations,etc. The location and degree of upper airway structure narrowing in patients have been investigated in many ways, such as X-ray, multi-slices spiral computed tomography, etc. With multi-planar reconstruction technique,3-dimensional construction of upper airway can be established which shows the delicate changes of the upper airway structure. Mandibular advancement device is known as an effective treatment on mild and moderate OSAHS. By advancing the mandible forward, it can increase the space of upper airway, especially the oropharyngeal space. This paper reviewed the methods of investigating on OSAHS and the change of upper airway structure in OSAHS patients treated with mandibular advancement device. Supported by Combined Research Fund of Bureau of Health, Yunan Province and Kunming Medical College(Grant No.2009CD205).