Objectives: To evaluate the upper airway changes before and after the use of Mandibular Advancement Device (OAm) of the patients with mild to moderate obstructive sleep apnoea hypopnea syndrome (OSAHS) using Cone Beam Computed Tomography (CBCT). Design: Prospective study recruiting patients from February 2015 to January 2017 Settings: Department of Stomatology at Yan’an Hospital Affiliated to Kunming Medical University, China. Participants: PSG confirmed Chinese OSAHS patients. Main outcomes measures: CBCT scans were done before and after the use of OAm to assess the change in the sagittal and coronal diameter, volumetric changes and the change in Minimal Cross-sectional Area (MCA) of different segments of upper airway. Results: A total of 30 patients were included with an average age of 49.53 ± 6.62 years. A significant increase in sagittal and coronal diameter of the lower margin of soft palate and the upper epiglottis was observed except the hard palate plane after OAm (P>0.05). MCA and volume of the posterior soft palate, lingual, epiglottis area and total airway were significantly increased with the use of OAm (P<0.001). Conclusion: OAm is a beneficial device in OSAHS patients as it increases the upper airway area and the volumetric changes thereby reducing the effects of OSAHS.