Because web services provided by different providers may be different in Quality of Service (QoS), so it needs to select web services for composite service dynamically. This paper gives methods that are built on the base of QoS description and addresses the issue of selecting web services for the purpose of their composition. The effect of service level and workload on QoS is illustrated. Based on web service QoS model, several methods for dynamic service selection are discussed. Experiments are conducted to evaluate the performance of these methods.