Many chemical processes can be operationally risky, environmentally harmful and potentially dangerous when abnormal or destructive situations occur. This is due to the fact that chemical processes are often operated under high pressures, at high temperatures, and with fast material flows and complex manufacturing mechanisms. In the extreme, catastrophes such as explosions, toxic release and loss of life will occur unexpectedly and rapidly, particularly when a terrorist who has a sufficient technical background in chemical operations attacks a plant. To assure process security, systematic and effective process security-bearing operational strategies must be developed. This paper introduces a process operational space classification method and a process operational security index. The method and the index can be an effective quantitative tool for characterizing and analysing process security, especially when the process experiences various disturbances set by saboteurs who may be technically very knowledgeable. The efficacy of the quantification method is demonstrated by its application to an exothermic batch reactor.