This paper investigates a concept of model-less trajectory-based stability, which is defined just by the characteristics of the on-line trajectory. With reference to an existing heuristic engineering method called the Extended Equal Area Criterion (EEAC), a model-less trajectory-based criterion for the system behavior stability is presented. This theory is potentially useful to the problem of assessing the system stability in emergency control. This paper investigates a concept of model-less trajectory-based stability, which is defined just by the characteristics of the on-line trajectory. With reference to an existing heuristic engineering method called the Extended Equal Area Criterion (EEAC), a model-less trajectory-based criterion for the system behavior stability is presented. This theory is potentially useful to the problem of assessing the system stability in emergency control.