A significant challenge of successful application of the Service- Oriented Architecture (SOA) in large-scale distributed systems is the Quality of Service (QoS) management, which provides various QoS guarantee levels for concurrent clients through effective resource allocations and adaptations. In this paper, we propose a policy-based approach for specifying QoS management strategies and enforcing QoS guarantees. This approach enables easy adaptation of new business rules and adaptation to system resource changes. This approach is also effective for supporting QoS management, as demonstrated in our experiments in a Publish/ Subscribe system.