Workflow access control is an important issue in workflow security. Although there exist some standards for security of Web services and access control for services in distributed systems are well studied, there is a lack of comprehensive approach in access control for web services, especially in workflow. This paper proposes a service oriented workflow model and its access control model. The basic authorization unit is task service which is an abstraction of task in the access control model. In this context, we present the dynamic separation of duty based on task services and their conflict relations. Furthermore, we show this model satisfies access control requirements in workflow.