The recording accuracy of Sirognathograph (Siemens, West Germany) was conducted by means of an experimental apparatus. Diagrams of mandibular border movements in three dimensional directions were performed. The results showed the distortion was not recognized within 5 mm of primary condition in a subject, but slightly between 5 mm and 10 mm. In a distance of more than 15 mm, a considerable distortion was increased until at the maximum opening position.