The emergence of Web Service has led to more research into Web Services composition. It has received much interest to support business-to-business or enterprise application integration and it will reduce development time and effort for new applications. But very little work has been done on the accurate and automatic Web Services composition. We first analyze the limitations of WSDL and DAML-S and point out why they don't fit to the accurate composition. Elicited from DFD, we propose a new Web Service composition model based on the global data schema and have designed the structure of the global schema. We discuss the composition algorithm in this new model. The data type matching algorithm is given, which is the core of accurate composition. Our implementation gives a whole idea of the composition model. Keywords. Web Service Composition, Global Data Schema, Data Type Matching, WSDL