Abstract: Objective To compare the clinical effects of auxiliary expansion arch and modified horizontal palatal bar on maxillary expansion. Methods 35 patients were randomly divided into two groups for expanding the maxillary dentition. One group used auxiliary expansion arches and the other used modified horizontal palatal bars. The changes in dental arch widths were compared after the therapy. Results In the auxiliary expansion arch group, the dental arch width was significantly increased between second premolars and first molars (about 4 mm), but not between the first premolars (about 2 mm). While in the modified horizontal palatal bar group, the dental arch width all significantly increased about 4 mm between first and second premolars and between first molars. Conclusions The auxiliary expansion arch and the modified horizontal palatal bar are both effective for expanding maxillary dentition in mild-moderate dental crowding patients. Clinicians may choose appropriate method according to patients’ characteristics.