Recent development in interventional radiology enables minimally invasive treatment of aortic aneurysm: endovascular stent grafting. In this paper, we report image processing method in preoperative planning and intraoperative guidance for assisting endovascular stent grafting. In preoperative planning, it is necessary to design stent graft suitable for each clinical case. Thus we present shape measurement method of aneurysmal aorta for computer assisted design of stent graft. In stent grafting, fluoroscopic image is used as intraoperative image for visualizing lesion and interventional device. As ftuoroscopic image is a two dimensional (2-D) image, a doctor has to imagine three dimensional (3-D) shape of aneurysmal aorta. For intraoperative guidance, it is useful to register preoperative 3-D CT image with intraoperative 2-D fluoroscopic images. In this paper, we present intraoperative 3-D/2-D registration method for assisting endovascular stent grafting and show application result to clinical data.