Quality of Services (QoS) management has become an important issue for Web services. Indeed, QoS is becoming a crucial and a distinguishing criterion among functionally equivalent Web services. QoS management consists of two complementary tasks: monitoring and adaptation. Both are very challenging because of the unpredictable and dynamic nature of Web service composition. In this paper, we propose a broker based architecture that enables dynamic QoS monitoring and adaptation for composite Web services. Our approach consists of dynamically changing the execution paths of composed Web services by instrumenting the BPEL process. We developed a prototype to evaluate the QoS adaptation scheme using our architecture and the result showed that the QoS of the BPEL process has been considerably adapted and improved comparing to the original one.