As the Web is becoming a medium widely used as a preferential channel for critical information exchange, business, and e-commerce, it is necessary to enable differentiated service mechanisms not only at the network but also at the Web server level. In this paper, we present a new service differentiation mechanism (Arrival-Related Dynamic Partitioning) for cluster-based network servers. Through the simulations, we demonstrate it can guarantee that customers with higher priority classes receive better services than ones with lower priority class and achieve the basic goals with service differentiation, especially when the system is heavily loaded and no enough server resources to allocate. And the admission control mechanism is also provided to prevent a Web site from being overwhelmed by excessive user requests.