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. 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.