Objective: The purpose of this study was to investigate the errors in a computer-aided design (CAD) method in the unidirectional mandibular distraction osteogenesis (DO).Methods: Six patients with mandibular ramus hypoplasia were selected and followed up.Computed tomography (CT) scans were performed at two time intervals: before surgery and at the end of consolidation.The mandibular DO plan was designed using CT-based three-dimensional visible software.The osteotomy line and drill location were transferred to a rapid prototyping surgical guide.The osteotomy of the mandible and implantation of the distraction device were completely under the guidance.