Web service composition has been recognized as a flexible way for resource sharing and application integration. For Web services providing similar functionality, Quality of Service (QoS) is the main factor to differentiate them. In this paper, the issue of QoS driven service selection in dynamic Web service composition is investigated. OWL-S is extended with a flexible QoS model to better facilitate service selection process. Based on different global constraints users set, the QoS-based service selection problems for composite services are divided into three categories. For each category, the mathematical model and its corresponding solving algorithm are put forward separately. Simulation experiments are conducted to show the validity and efficiency of the algorithms.