Aiming at a clinical case of a 52-year-old male patient, the patient’s mandible will be 3 Dimensional (3D) reconstructed and the surgery will be planed. This patient who suffered from mandibular tumor needed to undergo surgery. This study used a Toshiba 64-row Spiral CT to scan patient’s maxilla and mandible. 368 layers consecutive maxillary and mandibular CT images which scanning slice thickness is 1mm were obtained. The original images which format were DICOM were import to Mimics. After decided the bone threshold value the contour lines of every layers were extracted. Every image was passed the processing steps of edge division, selective editing, filling holes, wiping off redundant data and 3D reconstruction. After these steps the 3D geometric model of maxilla and mandible was obtained. The dimension of mandibular tumor and accurate location of the removal part were determined. These steps provide accurate original data for the manufacture of mandibular Rapid Prototyping (RP) model.