The product data exchange between heterogeneous CAB and PDM systems is a crucial issue for the integration of product development systems. STEP offers an efficient mechanism of product data exchange between heterogeneous systems. This paper introduces a UML-based mapping methodology for the product data models. The suggested mapping method has been applied to exchange the product structure data between CAD and PDM systems. Based on the STEP methods, we developed an interface module between a CAD system and a PDM system.