The treatment of class II division 1 malocclusion with retruded mandible in the mixed dentition period is suggested to use functional appliance. Bionator is one of the removable myofunctional appliance which is less bulky and simpler design compare to the activator, so that bionator can be worn all the time. An 8 year, 9 month old girl patient who has class II division 1 malocclusion with retruded mandible, overjet 11 mm, overbite 7 mm, and convex facial profile with lip trap and lip incompetence. Bionator has been used for about 8 month. The results were satisfied, normal overjet, overbite, and a pleasant appearance. The successfull treatment of this case, beside patient in the growing period, patient should had a good cooperation.