The goal of process planning is to convert design specifications into manufacturing instructions to make products within the specifications at the lowest cost. Therefore, for a computer-aided process planning system (CAPP) to generate a feasible and economical process plan, the tolerance information from design and manufacturing processes must be carefully studied. The geometric tolerances are usually specified in design only when higher accuracy of a feature (such as flatness, roundness, etc.) or a relationship (such as parallelism, perpendicularity, etc.) is required. For the relationships with dimensional tolerances or geometric tolerances with specified design datum(s), the selection of manufacturing datum and setup in process planning plays a very important role to make parts precisely and economically. This paper presents a neural network approach for CAPP to automatically select manufacturing datums for rotational parts on the basis of the shape of the parts and tolerance constraints. A back-propagation algorithm is used and some experiments are conducted. The results are analyzed and further research is proposed.