1. A recommendation method comprising: providing an ontology database, wherein the ontology database comprises a plurality of entities, the entities are arranged in an ontology hierarchy structure with N hierarchy levels {L_i}, i=1, 2,. .. , N, and N is an integer; storing, through the ontology database, a plurality of j th user data respectively corresponding to a plurality of users, wherein each of the j th user data records at least one j th entity of the entities on the j th hierarchy level L_j; generating a plurality of k th user data corresponding to the users according to the j th user data respectively, wherein each of the k th user data records at least one k th entity of the entities on the k th hierarchy level L_k; clustering the k th user data; and recommending the entities in the ontology database to the users according to the clustering result.