With the increasing diversity and complexity of distributed systems in network environment, web application server (WAS) is required to transform its service model from Best Effort?to QoS (quality of service) Guarantee". However, most existing WASs are relatively weak in terms of QoS provision. In this paper, OnceAS/Q is presented to show how a QoS-enabled WAS can offer different QoS provisions for applications. OnceAS/Q provides a set of QoS related services and a framework to support QoS guarantee, in order to realize a QoS enabled WAS. This paper first describes the architecture and main components of OnceAS/Q. Then two key issues, related to QoS enabling and the corresponding solutions, are explored in detail: one is the definition and mapping of QoS specifications, and the other is the dynamic reconfiguration of QoS-aware service components and resources. Ecperf benchmark, a well-known performance benchmark for J2EE Application Server, is used to evaluate the effect of QoS enabling of OnceAS/Q. Experimental results show that OnceAS/Q can support better QoS for applications at a reasonable cost.