This study was made with lateral cephalometric radiography of 28 skeletal class III malocclusion patients that were performed to setback surgery of mandible. The 28 patients were selected by four standards as follows. 1) Set-back amount of mandible is below l0 mm 2) No extrusion and intrusion of posterior tooth or alteration of interincisial angle at period of postoperative orthodontic treatment. 3) Change of mesial segment location of mandible on lateral cephalometrics 4) No genioplasty And 28 patients were divided to three group(1,2,3 group) by degree of preoperative occlusal plane angle to Burstone's horizontal plane. The preoperative occlusal plane angle, which of 1 group was smaller than 7 degrees and 2 group was between 7 degrees to 15 degrees and 3 group was larger than 15 degrees. The results were as follows: l. As the preoperative occlusal plane angle was larger, the degree of mandibular prognathism was not severe 2. On comparsion of preoperative and immediate postoperative cephalometric analysis, specific relationship of occlusal plane angle and set-back amount of mandible was not present. 8. As the preoperative occlusal plane angle was smaller, the alteration of postoperative occlusal plane angle was increased tendency. 3. As the preoperative occlusal plane angle was larger, the alteration of postoperative occlusal plane angle was decreased tendency. 4. The relapsed degree of B point distance to Vertical plane was not relationship to the degree of preoperative occlusal plane angle.