Introduction: Velopharyngeal inadequacy (VPI) may result in inappropriate oral/nasal coupling during the production of speech sounds, resulting in unwanted nasal resonance and/or nasal air emission. Orthognathic surgeries such as maxillary and/or mandibular advancements are also known to change skeletal and muscular structures resulting in changes affecting velopharyngeal closure (VPC). Although many studies have reported on the effect of maxillary advancement surgery on VPI for patients with cleft lip and palate, the effect of mandibular advancement on VPI has not been studied at length. The purpose of this study was to elucidate the effect of temporary prosthetic mandibular advancement on velopharyngeal function. Methods: Fourteen subjects (7 males, 7 females) with no history of craniofacial abnormalities or speech disorders were recruited. The mean age was 35 years (range = 26-60). Acoustic nasalance measurements were obtained during nasal sentences and during sentences without nasal consonants in two conditions; normally, and while wearing an elastic mandibular advancement (EMA) appliance to advance the mandible by 13mm. In addition, subjects were asked to produce five repetitions of the sentence "Ten men came in when Jane left" while recordings were obtained with a videoendoscopy/phototransducer system that sensed the amount of light passing through the velopharyngeal orifice. The endoscope and fiber optic light were inserted through the subject's middle nasal meatus and positioned above the velum. The phototransducer fiber was extended through the velopharyngeal port into the upper oropharynx to detect light passing through the orifice as the velopharyngeal mechanism opened and closed. Individual subject's outcomes with and without the EMA appliance were analyzed statistically using paired t-test for Nasalance test, and one-way ANOVA/independent samples t-test for phototransducer test. Results: Nasalance did not deteriorate, but significantly decreased for the ‘nasal’ sentences after mandibular advancement, whereas changes in nasalance were not significant for the sentences containing no nasal consonants after mandibular advancement. Mandibular advancement by a 13 mm using an EMA appliance did not significantly affect VPC. Instead, large variability among subjects in response to mandibular advancement. For 7 of the 14 subjects, the extent of VPC decreased significantly (p < .05) under the advanced mandible condition compared to the normal condition (without the EMA appliance). On the other hand, 5 subjects showed significantly (p < .05) increased VPC when their mandibles were advanced. For 2 subjects, VPC was not significantly changed with the advanced mandible. Conclusions: The outcomes of this study suggested that there was no statistical evidence to support that nasality was deteriorated by a 13mm mandibular advancement, which agreed with recent studies describing velopharyngeal function and nasality after orthognathic surgeries. VPC was not affected by mandibular advancement. Responses of the nasalance and VPC to mandibular advancement were dependent on the individuals. Further investigation such as electromyography method is needed to understand how velopharyngeal function and speech respond to mandibular advancement more definitely.