The maintenance of mandibular position is an important part of the treatment process in the hemimandibulectomy patient. It corrects the mandibular position into the normal and helps patient function the jaw. This case report presents the usage of an oral appliance modified from the upper occlusal splint, which the palatal and labial flanges were added for guidance in a 31-year-old female patient who was treated for myxoid chondrosarcoma by the right hemimandibulectomy. After the surgery, the patient had her mandible deviated 10 millimeters to the right in the habitual occlusion. By wearing the oral appliance at night time for 6 months, she could constantly bite in the intercuspal position without mandibular manipulation. The patient reported that her mastication was improved. In conclusion, the intraoral appliance modified from the occlusal splint is profitable in maintaining the mandibular position of the hemimandibulectomy patient.