The pathologic conditions that increase nasal airway resistance(NAR) have an effect on the craniofacial development. Bilateral expansion of the maxilla is sometimes desired in orthodontic treatment in order to coordinate the widths of the maxillary teeth to the mandibular teeth for the correction of posterior crossbite. Many studies showed improvement of nasal breathing after rapid maxillary expansion subjectively. The purpose of this study was to investigate the effect of rapid maxillary expansion on NAR objectively. For 18 subjects(9 males and 9 females), recording of NAR were taken prior to expansion, 1 month, 3 months, and 6 months retention period after rapid maxillary expansion. The results showed reduction from 3.84 to 3.02 mmH₂O/LPS in NAR(p<0.05). The reduction of NAR appeared stable during the retention period. From these results, it was suggested that rapid maxillary expansion is a useful method to correct posterior crossbite and to reduce NAR simultaneously. Therefore, we must examine the dental status of the patient who complains nasal obstruction, and may consider rapid maxillary expansion for the patient who has nasal obstructuon combined with dental malocclusion.