Grid computing is an emerging computing paradigm that will have a significant impact on the next generation information infrastructure. Computer networks and the Grid should be integrated together for supporting high-performance applications and a QoS-based network service broker is the key to this integration. In this paper, we propose a new approach for describing QoS capabilities of network services and develop a QoS-based broker technology for selecting network services that meet QoS requirements of Grid applications. Our approach for network service description and selection is highly flexible and applicable to networking systems with various implementations. We also propose a system architecture for realizing the QoS-based network service broker within the service-oriented Grid architecture.