Abstract: STEP AP 203 (ISO 10303-203-1994) is the international standard for the representation of product model data for configuration controlled design of 3D mechanical parts and assemblies, that offers an efficient neutral mechanism of information communication, independent from any particular system. However, the objective of interoperability cannot be completely achieved unless systems can be tested to verify whether they conform to the relevant product data exchange standards. This article introduces a systematic approach to implement the conformance testing of STEP and illustrates an AP 203 conformance testing software as an example.