XML is rapidly emerging as the dominant standard for exchanging data on the WWW. Most of application data are stored in relational databases due to its popularity and rich development experiences over it. Therefore, how to provide a proper mapping approach from relational data to XML documents becomes an important topic. Referential integrity constraints are useful for semantic specification that plays the important roles in relation schema definition. So far, there are about a dozen of XML schema languages have been proposed. Although most languages are based on DTD, they have different grammar resulting in the XML world more complicated. In this paper, we investigate referential integrity constraints for XML. An extended DTD with several kinds of constraints-DTD_c is proposed, and a corresponding equivalent mapping approach, ICMAP is presented. Finally, the comparison on the respect of constraint representation capability is performed, which proves that ICMAP is more effective than other known approaches.