The organizational structures-based workflow participant definition, which appoints activity executors according to user?s role, department or ID, is widely used in commercial workflow management systems presently. This approach lacks of flexible mechanisms for complex participants assignment. This paper described a novel approach (ARDE) to define workflow participants. ARDE defines participant from four facets, namely Assignment, Rule, Duration and Exception. Assignment uses an SQL-like language to define the qualification that activity participants must meet. Rule expresses a regulation to choose user(s) from all of qualified participants to undertake the task at runtime. The regulation considers the use?s context, including workload, online and vacancy, etc. Duration is the interval during which the task starts and completes. Exception represents the handling strategy when a task cannot be finished in the planned duration. In order to enforce the expression capacity, an organizational meta-model, several built-in functions and an entity outdata are introduced in this article.