XML is emerging as the dominant standard for information exchange and data representation over the Web. The modeling ability of object-oriented methods is strong, such as inheritance, nonmonotonic multiple inheritance, polymorphism, complex data structure, etc. Extending XML with some object-oriented features can improve the modeling ability of XML data model. DTD is extended with element hierarchy, multiple inheritance, overriding, blocking, polymorphism and conflict handling and XML-RL is extended with polymorphic element as well as inclusive element. XML-RL is a rule-based XML query language and it is based on high-level data model. The syntax and semantics of object-oriented XML database are described systemically.