Service-Oriented Information Technology is a new emerging paradigm. It can drive the development with rapid, low-cost and easy composition of distributed applications in software product. However, it has several problems such as how to identify service and determine service granularity. This paper proposes a solution that identifies service by using ontology in product line. We analyze service identification methodologies and approaches, and then define a service of feature model from various extensions of Feature-Oriented Domain Analysis. To compensate for ambiguity and expression, ontology is used to precisely define relation and type among features. A serviceoriented method for service identification using ontology can identify a service easily and quickly with proper granularity. This method also provides a way of grouping features and evaluating service after refining service candidate. This service-oriented method can make service with suitable granularity, and improve reusability of a service unit for identified service in software product line development.