Introduction: The aim of this study was to evaluate the effects of the Class II activator and the Class II activator high-pull headgear (HG) combination on the mandible with 3-dimensional (3D) finite element stress analysis. Methods: A 3D finite element model of the mandible was constructed from a dry human mandible. To investigate the effects of the Class II activator, a 3D model of the lower part of this appliance was constructed and fixed on the mandibular model. The Class II activator high-pull headgear model was established as described, and an extraoral traction force of 350 g was directed from the middle of the Class II activator to the top of the mandibular condyle. The stress regions were studied with the finite element method. Results: The regions near the muscle attachment areas were affected the most. The inner part of the coronoid process and the gonial area had the maximum stress values. Conclusions: Both functional appliances can cause morphologic changes on the mandible by activating the masticatory muscles to change the growth direction.