Objective: To examine adaptive changes in the shape of the upper airway during titrated mandibular advancement. Furthermore, to understand the mechanism of mandible-adjustable therapy to improve the ventilation., Methods: Fourteen non-apnea participated in the study. A custom-made oral device was used to keep the mandible in titrated advancement at 0% (F0), 50% (F50), 75% (F75), 100% (F100) of the maximum advancement. Magnetic resonance imaging was used to investigate the difference in the shape of upper-airway in these 4 positions. Changes in the anteroposterior direction (AP), lateral direction (Lat) and its ratio (AP/Lat) were calculated, which were transformed into the percentage of the original ones., Results: The dose-dependent decrease of AP/Lat was found when the mandible was advanced (P = 0.0001). Lateral change rate in percentage increased by degrees (P = 0.0023), while the increase of anteroposterior change rate in percentage showed no statistical significance., Conclusion: The shape of upper airway tended to be less round. The enlargement of upper airway during titrated mandibular advancement was mainly the result of enlargement in lateral direction.