We set a new basic plane on an orthopantomogram in order to measure the gonial angle and obtained the following: 1) Usable error difference in ordinary clinical setting ranged from 0.5 degrees-1.0 degree. 2) Because there is no great difference between the levels of the gonial on orthopantomograms, they are effective for investigation of the changes of side and degree. Thus, based on the above results, 100 cases were randomly selected from the Angle class III cases which were treated at our department of orthodontics in order to investigate the differences between the left and the right side and pre and post-treatment changes and obtained the following results: 1) Pretreatment differences between the right and the left sides were recognized in 88 cases. 2) 66 cases showed posttreatment improvement in the differences between both sides. Because orthopantomograms are applied at the end of treatment and for evaluation of the stability of occlusion. We will investigate if the occlusion is best harmonized at equal gonial angles on both sides, using a combination of EMG and measurement of occlusion force, and will also study various cases in more detail.