Many issues about portability and extension of system converting and exchanging information using XML standard data are being studied currently. This paper proposes design using XML processing layer for mediation between client and server to solve the development cost problem for compatibility and extension of client/server environment. This maintains mediation role for programming development between client and server and increases extension of system and portability of hetero geneous environment. This paper implements and designs XML data processing system for EC using XML DB among hetrogeneous databases. We expect cost-down by maintenance of system using this architecture for electronic commerce.