Synchronization between real-time ultrasound (US) and preoperative images can provide much information for US-guided intervention. For the synchronization, we present a real-time registration system between the two images of the liver without any help of sensors. In this system, we first generate a 4D preoperative image, which is composed of multiple 3D images along the respiration, by considering their local deformation. In the intraoperative stage, we achieve the pose information of a pose-fixed 3D US transducer by using several 3D US images. We then acquire 2D US images and find their corresponding images in realtime from the 4D preoperative image. The related registration is done by comparing a gradient-based similarity measure between a 2D US image and generated 2D preoperative image candidates. By the visual assessment of registration results, we confirm the feasibility of the proposed system for image-guidance.