Objective: This study was carried out to determine dental and skeletal effects of the Jumper twin block appliance in the treatment of developing Class II, Division I malocclusion. Methods: 15 girls aged 9 to 11 years old with class II division 1 and same physical maturation status were included in the study over one year. Cephalometric data and study models were analyzed statistically with Paired t-test. Results: The Jumper Twin block had skeletal and dental effects. Stimulation of forward mandibular growth, backward displacement of the maxilla, significant increase in the anterior and posterior facial height, retroclination of the upper incisors, proclination of the lower incisors and significant improvement of the overjet and overbite had been found. Conclusions: The Jumper Twin Block appliance with gradual bite advancement was effective in treatment of developing Class II division 1 malocclusion.