Directed surface extraction from CT images is the first task in the design of medical equipment. In this paper a new approach based on level set method is proposed to extract the directed surface from CT images. Two level set functions with corresponding speed functions are involved in this study. One is used to cut the desired bone from the input CT model in which the directed surface, usually the outermost surface, and the complex inner surface are both contained. The other is used to remove the complex inner surface. The experimental results show the feasible of the proposed method.