Abstract: Establishment of an assembly model capable of representing function-structure relationship and free from detailed geometry is one of key issues for supporting conceptual design in CAD system．This paper makes an attempt to establish a logic interrelationship between functional requirements and geometric embodiment construction in terms of the definition of logic assembly unit and discusses the form of functional description and geometric embodiment representation respectively．The representation formalism of logic modeling and assembly modeling processes are presented and an assembly template definition for supporting top-down conceptual design is correspondingly outlined．The proposed scheme contributes to provide elementarily a framework for developing computer aided conceptual design system．