QoS, that defines service quality such as latency, availability, timeliness and reliability, is important for web applications that provide real-time information, multimedia content, or time-critical services. Many web applications require a guaranteed server processing capacity. In this research, we study the QoS control issues using the current web services standards. We propose a QoS web service architecture, QCWS, by deploying a QoS Broker between web service clients and web service providers. In the QCWS architecture, a QoS Broker collects the QoS information about service providers that may offer qualified web services to a client, makes selection decisions based on some cost criteria, and negotiates with servers to secure QoS service commitment from one of them. We have implemented a QCWS prototype using IBM WSDK, enhanced with simple QoS capabilities. We have also measured the performance running under different service priorities.