OBJECTIVE To study changes of dental and palatal arch forms between adults and children after rapid maxillary expansion (RME). METHODS Trans-arch and trans-palatal widths, and molar and palatal angles on plaster models of 47 children and 47 adults were measured pre- and post-treatment with RME. RESULTS 1. Trans-arch width: Increases of trans-molar and trans-cuspid widths were significantly greater in the child group than those in the adult group, except first and second premolars. In the child group, there was no significant difference of increased widths between posterior teeth and cuspids. But, in the adult group, increased widths in trans-arch of first molars, first and second premolars were significantly greater than those in cuspids. 2. Transpalatal width: Trans-palatal increases at mid-palates and the apex of palates were significantly greater in the child group than those in the adult group. 3. Molar angle: In the child group, no difference existed in the molar group between pre- and post-treament. But, in the adult group, statistical difference existed between pre- and post-treatment. Prior to RME, the molar angle was the same between the child group and the adult group. The molar angle was significantly greater in the adult than that in the child group after RME. 4. Palatal angle: The changes were not of statistical significance in child palatal angles between pre- and post-RME. Significant differences existed in the adult group between pre- and post-RME. Increases of palatal angles were significantly greater in the adult than those in the child group. CONCLUSION Significant differences existed between the child and the adult group in forms of dental arches and palatal arches, which focused on the changes of molars and palatal angles after RME.