Object-oriented methodologies have been applied to developing internet applications for increase reuse and expandability, However, supports of object-oriented methodologies for web-based user interface system are insufficient. This paper provides an object-oriented user interface system design methodology for reducing time and effort required to design and develop user interface system of web-based application and for increase reuse of it. In this study, UML, a standard object-oriented modeling language is used for the object-oriented design. The proposed methodology is that user interface objects are extracted at object modeling phase and usecase description is presented for user interface system design. After preparing of usecase diagram from the usecase description, user interface class diagrams are presented which are integrated web-based user interface objects and user interface usecases on the usecase diagram. The merits of the method are that user interface objects can be reused for the View model of MVC model on similar web-based application at first, and it would be more easier to correct user interface system after design or development a system since standard object-oriented modeling concepts are used. Therefore, the efforts for user interface design, development and maintenance could be reduced with the proposed method.