Document Object Model (DOM) is an application-programming interface that can be applied to process XML documents. It defines the logical structure, the accessing interfaces and the operation methods for the document. In the DOM, an original document is mapped to a tree structure. Therefore ，the computer program can easily traverse the tree manipulate the nodes in the tree. In this paper, the fundamental models, definitions and specifications of DOM are surveyed. Then we create an experimenta1 system of DOM called XML On-Line Parser. The front-end of the system is built by the Web-based user interface for the XML document input and the parsed result output. On the other hand, the back-end of the system is built by an ASP program, which transforms the original document to DOM tree for document manipulation. This on-line system can be linked with a general-purpose web browser to check the well-formedness and the validity of the XML documents.