The basic concept of semantic Web,ontology and semantic annotation are described.Then the semantic annotation technology and tool today are introduced and analyzed,and a way of automatic semantic annotation based on HTML documents that contain rich semantic data on the Web is presented.This method couples structural analysis of documents with semantic analysis incorporating domain ontologies and lexical database Hownet,discovers the semantic partition tree corresponding to documents,and annotates HTML documents with semantic lables.The experiment is based on the HTML documents of electronic products,the result shows the method is feasible.