Abstract: Formal representation of spatial relations of assembly is a foundation for spatial semantic inquiry, analysis and assembly planning of products.Based on the characteristics of mechanical product assembly, the spatial topology and directional relations in assembly are analyzed.The spatial adjacency relations, inserting relations and separating relations between assembly bodies are proposed.Based on morphological taxonomy and topological description logic theory, a formal representation for spatial relations integrating topology and direction are given.By means of three-dimensional solid boundary geometry processing, the atom relations in formal representation are defined.Based on semantic reasoning the complex spatial relationships between assembly bodies can be automatically constructed and annotated.Ontology-based spatial relationship's RDF/XML document can be obtained.Finally, an injection mold assembly example for automatically constructing and annotating the spatial relationship is given.The results show that the proposed methods of semantic inference can be well used to realize the formal representation for spatial relations of assembly.