The purpose of this study was to evaluate and compare the dental effects of bonded rapid maxillary expansion (RME) and bonded slow maxillary expansion (SME) treatment methods. Subjects with a maxillary bilateral crossbite were selected and two treatment groups each with 12 patients were constructed. At the beginning of treatment, the average chronological ages were 11.96 years for the RME group and 12.31 years for the SME group. The Hyrax screw in the RME treatment group and the spring of the Minne-Expander in the SME treatment group were embedded in the posterior bite planes, which had a thickness of 1 mm. The treatment time for the RME group varied from 0.70 to 1.60 months and for the SME group 1.00-5.16 months. At the end of active treatment the appliances were worn for retentive purpose for an additional 3 months. Orthodontic casts taken at the beginning and end of treatment, and at the end of the retention period formed the material for the study. Increases in the transversal width between the upper molars, upper first premolars, upper canines, lower canines, and in the upper arch perimeter were obtained. The increase in the upper inter-canine width was found to be significantly greater in the RME group compared with the SME group. Regression analysis indicated that arch perimeter gain through the treatment could be predicted as 0.65 times the amount of the posterior expansion for the RME group and 0.60 times the amount of posterior expansion for the SME group.