In the irrigation facilities, the irrigation system is connected systematically, and thus, it agrees to the object-oriented concept. Since it is necessary to go through comparative evaluations and to devise several alternatives plans in designing the irrigation system, it will be very efficient to use the objects that contain the design data. In this study, the object-oriented methodology has been proposed to define the objects, which will be used in the design system of irrigation facility. Furthermore, as for the essential elements of the objects, concept of element objects is formulated. By employing this concept, appropriate element objects have been derided for the irrigation facility. Necessary data model for realization of the objects is examined and selected. And then, required elements for applying the selected data model to the irrigation facility will be proposed