Digital library is an important system useful as the fulfillment of information needs library collection. The Online Public Access Catalog (OPAC) is a feature available in digital libraries that lets you browse library collections. In this research, semantic based query expansion system is developed. The goal is to improve search results to be more relevant. Domain used is information technology where book data obtained from online book site and book publishing sites as much as 500 data. The stage is to build ontology according to the data obtained. The formation of ontology refers to the data of UMM Informatics Engineering curriculum which refers to the Association for Computing Machinery (ACM) Ontology is applied to find the connection between a user entered keyword. To support the search results used cosine similarity method. Cosine similarity is a method to calculate the similarity between search results with user keywords so as to generate search results ranking. Stages of testing the system using precision obtained a value of 65.6. This indicates that the system is capable enough to provide search results that are relevant to a user's search keyword.