The ontology technology is introduced into the assembly modeling to improve the automation degree of the assembly sequence planning, realize the information sharing, reusing and transferring in related activities of the assembly, and implement the seamless integration and mutual operation between the assembly sequence planning system and other heterogeneous systems. Aiming at the modeling demand of the assembly sequence planning, the assembly sequence planning-oriented and assembly object-based assembly ontology named web ontology language for assembly sequence planning (OWL-ASP) is constructed. The OWL-ASP consists of Properties ontology, AssemblyInfo ontology and AssemblySeq ontology and theydescribe the assembly properties, assembly information and assembly sequence of the assembly object, respectively. Based on OWL-ASP, the rules for assembly are depicted with the semantic web rule language(SWRL) to form the unified hierarchical system for assembly knowledge representation and realize the strict depiction and automated reasoning of the assembly knowledge. Effectiveness of the proposed ontologies and rules for assembly is illustrated through a practical example of ball valve assembly.