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 (DiffServ) mechanisms not only at the network but also at the Web server level. We present a new service differentiation mechanism (arrival-related dynamic partitioning) for cluster-based network servers. We demonstrate by simulation that it can guarantee that customers with higher priority classes receive better service than ones with lower priority classes and achieve the basic goals with service differentiation, especially when the system is heavily loaded with not enough server resources to allocate. An admission control mechanism is also provided to prevent a Web site from being overwhelmed by excessive user requests.