Biomechanical changes of the mandible from orthopaedic chin cup forces were investigated by means of finite element analysis. A three-dimensional model of the mandible including the temporomandibular joint was developed for stress analysis in the mandible. A chin cup force of 400 gf was applied at pogonion on the mandible in the direction toward the condyle. From the stress analyses, it was revealed that (1) uniform tensile stresses were produced at the outer borders of the mandible, (2) compressive stresses were induced in the centre of the mandible, (3) compressive stresses were generated widely on the surface of the condyle, and (4) stresses on the condyle were smaller in magnitude than those in the mandibular corpus. These results coincide with morphological changes of the mandible revealed in previous studies and, thus, indicate an association of stresses with remodelling of the mandible from chin cup therapy applied to adolescent patients with mandibular prognathism.