One of the major problems of modern Information Retrieval (IR) systems is the word mismatch word mismatch that concerns the discrepancies between terms used for describing documents and the terms used by the researchers to describe their information need. One way of handling the Word mismatch is by using a thesaurus, that shows (usually semantic) the relationships between terms. The main goal of this study is to design and build an automatic Arabic thesaurus using Co-occurrence technique that can be used in any special field or domain to improve the expansion process and to get more relevance documents for the user's query. Results from this study were compared with the traditional information retrieval system.