The developer and user of weapon system must define the requirements of weapon system for the satisfaction of stakeholders and complexity of modern weapons in the developing process of weapon system. The perfect requirements reduce risks of the developing system in the initial step. It is not only economy of time and money but also good for the project of developing system. But, the perfect requirement is very difficult because of contradictions between the user and the developer, and limited conditions. So, the structured method and the process is needed for developing requirements.