This paper analyzes the difficulties of basic primitives of geometric modeling systems currently in effect, which is met in the application of numerical control manufacture. On the bases of the analysis, the paper proposes the primitive designing rules for the integrated CAD / CAM systems. The paper is divided into four parts. The first part analyzes the defects of often-used primitives. The second part discusses requirements of primitive in the integration of design and manufacture, and proposes the basic rules and unified method of designing primitives. The third part introduces rules and basic principles of constructing primitives offered by the writer. The last part provides an example to show the advantages of the modeling rules.