Abstract: The procedure oriented IEC 1131 3 specification has poor capability to support the open architecture design and object oriented implementation in representation of the control flow of machine tool. In order to meet the new requirement, we extended the concept of Finite State Machine, and proposed a new hierarchical FSM method for representing the control flow of machine tool. In this paper, the organization, properties and formal definition of hierarchical FSM are discussed in detail. A kind of general structure based on hierarchical FSM representation of machine tool controller is described. Moreover, the problems related to representing openness and system implementation are also discussed.