Abstract: To compose an XML-based article, two source files, the bared article itself and the bibliography database file, have to be efficiently integrated. This integration involves citation from the article file, search on the bibliography database and finally association with the cited references to form the result article. All above operating processes are XML-based. In order to investigate the feasibility of integrated operations, we propose an open XML-based Article Composing System (XACS) and present a mathematical model of the transformation processing. With XACS, an author can freely compose an article with his favorite editors, efficiently utilize the citation service and automatically generate the desired document format, such as PDF, TEX, or other XML format. Four phases are developed to compose an article in XACS: editing phase, validation phase, association phase and presentation phase. To explore the validity of transformation processing, a formal mathematical model is presented . Although every XML document can be represented by a tree, to compose an article requires more than two XML documents. The corresponding transformation has to be operated over forests. We thus give a forest grammar to model XML documents and a Macro Forest Transducer with equality test (MFTET) to completely model the transformation processing. Finally, concatenation and composition properties are verified to assert the validity of MFTET.