The strategic value of Inter-Organizational Information System (IOIS), regarded as an automated information system shared by two or more companies, highly depends on its architecture to support agile business needs. The main requirements of this architecture for supporting business agility are flexibility and scalability, interoperability and integration as well as process-centric architecture and continuous business process improvement. We claim that the service-oriented architecture (SOA) approach, the latest in a long series of attempts in software engineering, is the best for meeting these requirements. In this paper, after investigating the compatibility of SOA features with IOISs requirements, we propose a service-oriented IOIS architecture composed of business service, application and integration layers.