There is a growing need for managing importance of entities in knowledge system in order to realize the full potential of knowledge. How to calculate the importance of entities automatically is the primary issue. We argue that importance of entities in knowledge is dynamic, the importance is changed along with the using of ontology; and different groups of user have different criteria of importance. In this paper, a novel weight assignment method which takes usage and structure properties of ontology into account is proposed. When considering the usage information of ontology, we analyze paths that are used to respond to queries; and use the frequency of entities included in the paths to produce the optimal weight assignment for the assumption of high importance of entities which included in paths that respond to queries. After get the initial weight of entities, a pervasion algorithm which considers the structure of ontology is used to compute the final weight of entities. Weight of a node is high if the node has many incoming links and the incoming links and nodes which these links are from have high scores. Experiment show effectiveness of this weight assignment method.