Conductive hearing loss, the second most common type of hearing loss, happens when there is a problem transmitting sound waves into inner ear, which will bring a lot of inconvenience to the patient in life and put the impaired person at a competitive disadvantage. Although conductive hearing loss could be relieved by hearing aids, the inconvenience of using these removable aids or the surgery trauma of the implanted ones should not be overlooked. Transversal maxillary deficiency and high palatal arches are often found in patients with conductive hearing loss. Some researchers have reported that there is close relationship between the maxillary contraction and hearing damage. Luckily, rapid maxillary expansion (RME) has been proved to be an effective treatment method for the transversal maxillary deficiency in clinical orthodontics for more than 40 years. Thus, we hypothesized that RME treatment would be a potential therapeutic method for conductive hearing loss in growing children with maxillary constriction.