A patient (Hellman dental age IIIA) who had anterior cross bite due to functional factor was treated with activator. Following results were obtained: 1) Anterior cross bite was crorrected 3 months after the initial application of the appliance. 2) Comparing pretreatment records with posttreatment, the main effects were labioversion of upper incisors, increase of upper arch length and downward-backward rotation of the mandible. 3) There was no damage on teeth and periodontal tissues and the patient had normal occlusion 1 year after the treatment.