This paper discusses a new simple methodology for defining software system requirements. We have developed a practical approach which we call FS (Flow Sketch) methodology. This methodology, based on the data flow concept, has been developed to provide the precise means of user's requirements. Actually, the user's requirements are presented in data form by particular format cards. Data are classified and the relationships between data are decided through brainstorming. Then, a requirement definition model is defined. FS methodology employs diagrammatic notation. This notation is suitable for the visual and the interactive description of the dynamic system data flow. As a result, misunderstandings of the software system between the software producer and software user will decrease.