Abstract: XML is emerging as the standard for representing data in the World Wide Web. If a data server can efficient(v store and query the XML documents, it will allow users to manage volumes of data effectivelv. This paper describes new storage strategies and a cost model for XML documents. We suggest two storage techniques for storing XML documents in a relational database system. Our storage techniques preserve the attribute of XML documents such as the hierarchical structure of documents by considering both XML DTD and XML instances. The cost model is extended to estimate the queries on the XML documents in a relational database system. We have evaluated the performance of queries for four different storage techniques including suggested storage models and existing storage models by our cost model.