Abstract: An I/O_WF_Net model based on Petri nets is proposed in this paper. In the I/O_WF_Net model, the activities of a workflow are abstracted as transitions and the inputs and outputs of an activity are abstracted as places of a Petri net, so the model is convenient for test cases generating. The modeling method of the components and structures of a workflow are described, an algorithm that transforms a workflow net constrained by inputs and outputs into the I/O_WF_Net model and the corresponding simplifying method are given.