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For successful navigation during total knee replacement, it is elementary to determine the hip rotation centre accurately in order to adjust the geometrically correct position of a knee implant with respect to the mechanical leg axis. Current techniques rely on markers pinned on the moving distal femur, but there are no detail reports about the influence of the femoral moving range on accuracy of mechanic axis of femur. To accurately define the femoral mechanical axis, a novel research based on the reconstructing femoral CT image was put forward. Five moving ranges of femur were selected according to human's kinematics' characteristic and the optical tracking system defines the hip center and mechanical axis of femur accordingly. To know which moving range can get the more accurate hip center, CT machine scanned the femur and reconstructed its 3-D model; In UG software, we picked up the mechanical axis of reconstructed femur as the "accurate" axis. The paper compared five tracking defined axis with "accurate" axis, then the moving range which determine more accurate mechanical axis can be obtained. However, the experiments results also indicated that the tracking hip center prefers to medial, distal and posterior compared the accurate femoral head center.