This paper presents the development of a part-build orientation system for rapid prototyping by considering the volumetric error (VE) encountered in parts during the building process. The methodology involves a primitive volume approach, which assumes a complex part to be constructed from a combination of basic primitive volumes. First, the VE in a number of primitive volumes such as cylinders, cubes, hemispheres, cones and pyramids is considered. Then, the VE in a parts made from a combination of these primitives is determined. The system graphically displays the VE at different orientation of any part and recommends the best orientation for the minimum VE in the whole part. This paper will illustrate the methodology by considering the case of the orientation problem of a cone and the parts made from different primitives.