A function-based geometric modeling approach for preliminary design is developed using an object-oriented programming technique. Preliminary design of engineering systems such as ships and aircraft is a process finding a design object which satisfies several required functions. In the process, it is important to support designers so as to manipulate design objects flexibly. Moreover, it is necessary to manipulate geometries of design entities in order to evaluate their, functions. In this paper, we propose a modeling method of these design objects and their geometry. The system cooperates with both the modeling subsystem of design entities and design parameters, and the geometric modeling subsystem. In the system, each modeling element is represented as an object using an object-oriented programming technique. The procedures for modeling elements are carried out by the methods defined in classes in an autonomous fashion, and geometries are automatically generated with the procedures which are defined in objects of design entities. This approach is effective and helpful in representing design knowledge and supporting the designer's manipulation of design objects. Finally, the prototype system implemented with the method is applied to the preliminary design of a ship's hull arrangement to confirm its validity and effectiveness.