Feature-based modelling has been considered as an indispensable tool for integrating CAD/CAPP systems. One of its advantages over conventional geometric modelling is the ability to associate not only geometric and topological information, but also form features, tolerances, material properties and other information which may be used during the process planning. This paper presents a STEP-based feature modeller (STEP-FM) for prismatic parts. High-level three-dimensional solid features are used as the basic entities for part design. The modeller relies on three main steps; ( 1) selection of the part base shape and overall size, ( 2) selection of the features to be added to or subtracted from the part being designed, and ( 3) providing information needed to de. ne feature size, position, orientation and other attributes such as surface finish, tolerances, etc. The designed part is then exported as a STEP XML data format ( according to ISO 2000). This. le can be used to integrate into CAPP/CAM systems without using a complex feature recognition process. An object-oriented approach is used in the definition and implementation of the product model. This approach overs advantages of incremental system development and reusability. An example is given to demonstrate the application of the developed modeller.