Purpose: The aims of this study were to evaluate the effect of a mandibular advancement device on oropharyngeal dimension in patients with obstructive sleep apnea (OSA) and reveal the predominate site of changes produced by mandibular advancement using computed tomography (CT). Materials and Methods: CT scans of 20 patients diagnosed with OSA were taken with and without the appliance. Three-dimensional changes in pharyngeal shape measured on cross-sectional CT images during two respiratory cycles after oral appliance insertion were estimated at five vertical levels using three variables: (1) lateral dimension, (2) anteroposterior dimension, and (3) cross-sectional area. Various parameters related to severity of OSA such as snoring volume, frequency, duration, and episodes; breathing pauses; oxygen saturation; Epworth Sleepiness Scale (ESS) score; and Apnea Hypopnea Index (AHI) score underwent comparative evaluation subjectively and objectively. Data were analyzed using the Student t test for parametric analysis. Results: A significant increase in the lateral and anteroposterior dimension of the pharyngeal lumen was observed at all five levels, but the mean change was greatest at the retroglossal level and smallest at the hypopharyngral level in both the lateral and anteroposterior dimensions. The cross-sectional area at all levels appeared to increase significantly, and apnea indices improved significantly. A significant decrease in snoring volume, snoring frequency, breathing pauses, snoring duration, snoring episodes, ESS score, and AHI score and a significant increase in oxygen saturation were found after treatment with the mandibular advancement device. Conclusion: Within the limitations of this study, CT was shown to be useful in evaluating treatment efficacy in subjects with OSA.