To integrate heterogeneous distributed systems, there exist various researches and developments for the purpose of its adoption into enterprise environment. However, when service-oriented technologies are applied, it is difficult to adopt directly existing software system development methodologies, because of the primary elements of service-oriented architecture such as service provider, service requestor and service repository. In this paper, we define a conceptual model for developing systems in service-oriented computing. The model involves procedures and guidelines based on the Use Case model that is elicited from a business domain in requirement analysis. We focus on how to identify and how to determine key realization decisions for each service. The benefit of the identification method based on the Use Case model is that the service has reflected the business of the domain well and is in its proper granularity. The assessment guideline can help the settlement of a problem that is to determine the implementation strategy.