A knowledge-based geometric modeling system for preliminary design was developed using object-oriented approach. Three types of knowledge for design, i.e. the model representing a design product including the geometric model, the procedures manipulating the design product and the knowledge managing the design process were integrated in the system. The model built up with object-oriented programming provides an effective and flexible design environment. Finally, the system was applied to the preliminary design of a ship’s hull arrangement to ascertain its validity and effectiveness.