The STEP-based system is developed the same as the object-oriented information system with the exception of language usage. The data modelling language adopted in the STEP-based system is EXPRESS, while object-oriented methodologies employ UML. The translation or mapping between EXPRESS and object-oriented methodologies plays a key role when implementing a STEP-based system. This investigation focuses on the mapping between these two languages while implementing a STEP-based system. Current research has not completely examined the capability of EXPRESS in dynamic modelling and the collaboration of UML and EXPRESS in software engineering. Static and dynamic modelling mapping are considered herein to corroborate how to naturally amalgamate these two languages. Diagrams in UML are employed to discuss the elements to be mapped to EXPRESS language throughout the development of the system. On the other hand, EXPRESS representation of these elements is discussed and mapped to UML notations. An illustrative example, product structure management in PDMS, demonstrates the effectiveness of the proposed mapping methods. The development of a STEP-based system can be shortened and made more systematic via the designed methods.