Identifying topics and concepts associated with a set of documents is a critical task for
information retrieval systems. One approach is to associate a query with a set of topics selected from a
fixed ontology or vocabulary of terms. The core idea of this research is using Wikipedia articles and
associated pages to make a topic ontology for this purpose. The benefit of this method is that Wikipedia is
an online free-content encyclopedia which is developed through a social process and kept current by the
Wikipedia community. In this paper the Persian Wikipedia has been analyzed in accordance to its articles
and the category link graphs to extract a Persian pseudo-ontology. Thereafter, the created ontology has
been applied through a query expansion algorithm to improve the performance of an information retrieval
system. Our experiments show that it is possible to improve the precision of the information retrieval
system by queries expansion based on Wikipedia.