A fundamental task on the Grid is to decide what jobs to run on what computing resources based on job or application requirements. Our previous work on ontology-based matchmaking discusses a resource matchmaking mechanism using the Semantic Web technologies. We extend our previous work to provide dynamic access to such matchmaking capability by building a persistent online matchmaking service. Our implementation uses the Globus Toolkit for the Grid service development. It also exploits the monitoring and discovery service in the Grid infrastructure to dynamically discover and update resource information. A schema translator is developed to translate the various formats used in a heterogeneous Grid environment into our resource ontology. We describe the architecture of our semantic matchmaker service.