A RFID system consists of hardwares, such as RFID readers, antenna, tags, and application which run on host computer. RFID middleware is software which connects hardware with application program. It communicates with readers to fetch data from readers and transfer the data to application layer.Because of different hardware specifications, there are no unified protocol commands to use with various kinds of RFID readers. In addition, cost is seriously concerned when RFID system is integrated with enterprise applications since current RFID readers which are already installed by each organization are from different manufacturers. The users are not expected to use same RFID models in their system. Therefore, the requirement for RFID middleware in current market rises tremendously. With RFID middleware, even if reader hardware is changed or new readers are added, application implementer can save much energy and cost to solve the connectivity problems. And application level can easily be adapted to these changes without being modified.This paper proposes a RFID middleware architecture which uses XML protocol to function as source of protocols for reader communication. At first, it identifies all basic technologies and concepts related with implementing RFID middleware such as serial and socket communication, protocols, ALE, application-level events, and the concise ideas of developing application interface, and so forth. Secondly, a RFID middleware architecture was proposed based on ALE specification. Thirdly, Besides of common requirements such as data filtering, integration and reader management, additional middleware functionalities are identified and classified into six categories such as location, quantity, time, direction, speed, and security, to satisfy some fundamental requirements in application fields such as Supply Chain System, Warehouse Management System or Retail management System. Finally, a RFID middleware prototype was developed to show the proposed architecture is feasible and provide enterprise application interface to satisfy fundamental requirements of applications. In prototype system ALE layer was also implemented. The prototype system conforms to ALE specification of EPCglobal and supports various RFID readers including AR400ⓡ, FirmSysⓡ, ED3100ⓡ and ER200ⓡ.