A new concept, design base axis of femur, is presented in this paper. The design base axis of femur,serving as the reference for the CAD of artificial hip jionts, is defined as the analogue line of the inner outline shape centers of the femur CT sections along the femur axis. The design base axis of femur is determined through the following procedures : First, the varying of the X and Y coordinates of the shape centers of the selected CT sections along the CT axis is respectively analogued; Then,the femur design base is determined according to the rules and relations of projection.