A semantic based approach for QoS driven service discovery is proposed to help clients select the currently best services matching their requests in a dynamic GGS (GIS Grid Services) Virtual Organization. Services are selected according to the functional requirements and dynamic Qualities of Services (QoS) requirements. This framework utilizes ontology to define the terminologies on GGS functions and dynamic QoS semantics. A Semantic Registry Service is designed, which accepts service registration with semantics and updates the QoS metadata dynamically. Furthermore, the discovery process is based on ontology reasoning and can choose better service fulfill the QoS request. We have built the prototype system with the Globus Toolkit 4(GT4).