Millions of users search daily for their needs using internet and other information stores, they search by writing their queries. Unfortunately, these queries may fail to reach to their needs, this fail known as word mismatch. One way of handling this 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 Local Context Analysis 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. This technique can be used in any special field or domain to improve the expansion process and to get more relevant documents for the user's query. Results of this study were compared with the classical information retrieval system.