Without detailed exact knowledge of a retrieval collection, most users find it difficult to formulate effective queries. In fact, most users may spend large amount of time formulating queries in order to obtain their desired result. A method to overcome this difficulty is to use query expansion that reformulates better query from initial query. In this paper we propose concept based query evaluation method using concept of class that retrieved from initial query. This concept is expanded through thesaurus. For efficiency evaluation of query expansion, we defined most critical value through a simulation and compared precision and recall each other.