Abstract: bjective　To evaluate the effectiveness of early treatment with the Twin-block appliance for the developing Class Ⅱdivision 1 malocclusion.Methods　ATotal of 30 children with ClassⅡdivision 1 malocclusion, 18(8 male, 10 female) out of which received treatmentwith theTwin-block appliance, the other 12 cases(6 male, 6 female) without treatment served as con- trol group. Cephalometric data were collected at the start and the end of the study and statistical analysiswere applied.Results　 Except the factor of growth, treatment with the Twin-block appliance resulted in reduction of ANB (1·55°), overjet(5·46 mm) and correction of molar relationship(4·07 mm). These changes were due to the change of Pg/OLp,Co/OLp and mandibular length. The change of A/OLpwas not significant. The skeletal effect contributing to the change on overjet and molar relationwere 58% and 78% respectively.Conclusion　Early treatment with the Twin-block appliance was effective in reducing ANB angle, overjet and correction of molar relationship. The changes were mainly profit from the favorable skeletal change of mandible, espe- cially from its length, while the effect on maxillary was not significant.

Key words: Twin-block appliance, early orthodontic treatment, Angle′s ClassⅡmalocclusion, cephalometry