Growing attention has been paid to semantic service discovery using ontology-based service description for locating desired services. The service description can be enriched by linking it to a more refined ontology-based specification which represents particular characteristics of the service, including rating. This paper proposes an ontology-based rating model for service quality. Services are rated for quality by a third-party organisation that assigns scores in terms of ontological values. The services will be retrieved by a matching algorithm that follows a flexible match approach and they will be ranked based on service consumers? preference criteria. Using ratings, service discovery can enable service consumers to accurately locate, with confidence, quality services that meet their requirements.