Jacob disease is a rare condition consisting of new joint formation between the coronoid process of the mandible and the posterior aspect of the zygomatic arch. Jacob disease and coronoid elongation are both clinical features of coronoid hyperplasia. It is usually accompanied by limited mouth opening. The etiology and pathogenesis of coronoid hyperplasia and Jacob disease are unclear. This paper reports a case of a 25 year-old patient who presented limited mouth opening. Clinical examination and clinical interview were performed as well as panoramic, transcranial, Waters projection and cone-beam computed tomography (CBCT). Analysis of the CBCT confirmed the hypertrophy of the coronoid processes of the jaw, bilaterally and the new joint formation between the coronoid process and the posterior aspect of the zygomatic arch. Although the Jacob disease is a definitely uncommon, it can be found through careftil examination and proper radiographic study using CBCT.