Abstract: This paper presents a formal approach of product development and manufacturing process modeling based on process decomposition and activity description for a virtual enterprise．There are many constraints in the relations among tasks which compose the process，such as enabling constraints and facilitating constraints．Each task can be decomposed into a series of activities．The activity，task，state and constraint are represented in enterprise ontology in which the semantics are provided by situation calculus．The organization model is established separately from the process model．Based on these process and organization models，we build a workflow system to support a virtual enterprise．An Agent model composed of activity model，intention model，skill model and companion model is put forward to implement the workflow management．An Agent federation which can be nested is constructed to enhance system flexibility．