Objectives: This prospective clinical trial evaluates the skeletal and dental effects of a new clear mandibular advancement appliance (CMAA) based on the Twin-block design. Materials and Methods: Twenty-Five class II female patients aged 8-12 years (average 10) with mandibular deficiency (SNB≤78) were selected and fitted with the CMAA. The average follow-up period was 13 months. Cone Beam Computed Tomography scans of the patients pre and post operatively were obtained to evaluate the three-dimensional changes achieved with the use of the appliance. Results: There was significant increase in the effective mandibular length with an amount of 5.2mm ±2.58mm, a decrease in ANB angle by -1.88 + 0.99 whereas SNB angle showed statistically significant increase by 1.92 degrees. Conclusion: The CMAA is an effective functional appliance for treatment of growing skeletal class II malocclusions due to retrognathic mandible, though the skeletal changes were not fully expressed due to the clockwise rotation of the mandible.