With the development of Internet technology

the Web Service is playing a more and more important role in satisfying the needs of large and growing community

this in turn demands that Web Servers meet the high QoS(Quality of Service) requirements of applications.In particular

Web Servers needs to deal with problems as highly concurrent requests

different clients with different QoS requirements

and so on.Therefore

how to improve the QoS of Web Servers has become a hot research topic.This paper presents a new proportional delay differserv-enabled model for heterogeneous Web Server clusters.The model describes the request arriving and serving process of cluster nodes by : queuing theory.The paper also puts forward two adaptive algorithms for request selection and dispatch

both algorithms are based on feedback control and each of them has a unique request selection and dispatch strategy.The experiment result is also given.It shows that the model presented here is suitable for heterogeneous Web Server clusters

as the clusters can achieve better QoS performance when these two algorithms are applied.