A high-precision navigation system for Anterior Cruciate Ligament(ACL) reconstruction surgery is presented. In this system, 3D CT data is used to visualize the structure of bones and optical localizer is used to detect the position of femur, tibia and surgical tools. Since CT images are acquired preoperatively and markers are fixed to bones intraoperatively, their relative position has to be initialized. To achieve this initialization, 2D/3D registration method between CT model and X-ray images was developed. The registration is performed by minimizing the difference between contour lines. As ACL reconstruction requires precision, three X-ray images taken from different viewpoints are used for the registration. Experimental registration