Abstract: ObjectiveThis study aims to evaluate the changes of the maxillary and mandible after treatment of Twin-block appliancein the Angleclass II division 1 patients by cone-beam CT and export the treatment mechanism of Twin-block appliance . Methods Thirty oneAngleclass II division 1 patientsin the growth period were selected and treated with Twin-block appliance. Cone-beam CT were taken before and after treatment to measure and analysis.Results The length of the mandibular ascending branch（Co-Go）and the mandibular body(Pog-Co)in the 31 patients were significantly increased (P<0.01). The distance of B-coronal plane and the B-water plane increased and were significantlydifferent(P < 0.01), suggesting that the growth of the mandible is forward and downward. There was a significant difference in the decrease of ANB, U1/SN and the increase of L1/MP(P<0.01).There was no significant change in the maxillary.ConclusionTwin-Block appliance can effectively promote the growth and development of the mandible and correct the Angleclass II division 1 malocclusion.

Key words: class II division 1 malocclusion, Growth modification treatment , Twin-block appliance , cone-bean CT