Ontology plays an important role on the Semantic Web. In this paper, we propose a method, AOIWD, of acquiring ontology information from Web documents. The AOIWD method employs data mining techniques combined with inference engine techniques to develop ontology. One key feature of AOIWD is that it employs association rules and inference mechanism to create association instances of ontology classes. Another key feature of AOIWD is that it employs clustering to create ontology class hierarchy. The method can discover more meaningful relationships and has less human intervention.