Since the 1960's an overwhelming amount of in-house and custom engineering software has been written. In the effort to reduce the cost of maintaining existing codes and producing new applications, the recent introduction of the object-oriented design approach has proved successful. At the same time graphical user interfaces are gaining in popularity to improve the usability and versatility of an application. This thesis investigates the application of the object-oriented approach to the design of a graphical user interface for an engineering design application. The development of an object-oriented graphical user interface for the NASA Engine Performance Program, a turbine engine design code is presented. The design of the new object-oriented graphical user interface for extensibility and re-usability is discussed. Design considerations for integration of the interface with procedural and object-oriented versions of the conceptual aircraft design program, ACSYNT, are explained. An existing PHIGS-based object-oriented graphical user interfacing framework is extended and built upon to develop the class structure of the interface. The class organization is presented in commonly used notation and described in detail. / Master of Science