Objective To investigate the therapeutical effect of rapid maxillary expansion (RME) in treating open bite due to thumb-sucking. Methods A sample of ten early permanent dentition patients with open bite due to thumb-ears, with the mean age of 11.63 years old. The anterior open bite was 10 mm or so. The patients underwent RME with 2 turns a day (0.25 mm per turn) until the lingual cusp of the upper molars occluding with the buccal cusp of the lower molars. Immediately before appliance insertion and at the end of active expansion, posteroanterior cephalograms, occlusal radiographs and casts were taken. Paired t test was used for statistical analysis. Results The midpalatal suture was expanded. The width of maxillary bases was increased. Bizygomatic width, maxillary width and maxillary first molar width were increased by 2.30, 2.30 and 4.85 mm in average respectively. Maxillary intermolar cusp width and maxillary intercanine cusp width were increased. The upper arch form was modified obviously, creating space required for teeth alignment. Conclusions RME treatment is able to expand the midpalatal suture and induce pronounced transverse changes at the skeletal level, which is an effective therapeutic procedure for open bite due to thumb-sucking of the early permanent dentition patients.