A method for estimating three dimensional (3D) position of knee joint in vivo was presented. Volume data of a knee joint were generated using magnetic resonance imaging (MRI) scanning of the knee. The volume data was rendered as the mean voxel values along the ray drawn from the focal point to each pixel of the image plane. The 3D positions of the femur and tibia were recovered by matching the rendered image with calibrated bi-plane x-ray images of the knee. To validate this method, in vitro experiments were carried out using a cadaveric knee. To define the local coordinate systems, 4 polymeric markers containing contrast media were attached to the femur and the tibia, respectively. The knee underwent MRI scan to generate volume data. Then, bi-plane x-ray images of the knee were taken at 2 different flexion angles. The relative position (6 degree-of-freedom parameters) between the femur and tibia was compared with that measured using a 3D coordinate measuring machine. The maxi-mum errors of the rotation and translation parameters were about 3.2° and 10.9 mm.