An expert system is developed for engineering design based on object-oriented knowledge representation concept. The design process is understood as determining design variables and their relationships which compose design model. The design model is represented as a network in the computer system using the object-oriented knowledge representation. The system built with the above concept provides the following abilities: 1) flexible model building and easy modification, 2) effective diagnosis of the design process, 3) supporting method for redesign, 4) a hybrid function with numerical computations and graphics, and 5) applicability for various design problems. Finally, it is applied to the basic design of a ship.