A recommendation method includes providing an ontology database, in which the ontology database includes a plurality of entities, and the entities are arranged in an ontology hierarchy structure with N hierarchy levels; storing a plurality of jth level user data respectively corresponding to a plurality of users; generating a plurality of kth level user data according to the jth level user data respectively; clustering the kth level user data; and recommending the entities in the ontology database to the users according to a clustering result.