With the increasing availability of information in electronic form, it becomes more important and feasible to have automatic methods to retrieve relevant information in the Internet. A deficiency of traditional information retrieval systems is that search terms are often different from those indexed by the systems. Thus, users ma either retrieve wrong information or miss what they really want. In this paper, e used an automatic query expansion based expansion based on term distribution to enhance the performance of information retrieval. Also this thesis proposed the method for setting the threshold according to area distribution in order choose additional terms.