It is very difficult for a designer to design an aircraft, because many design variables with complicated relationships are involved in the aircraft design process. This is one of the reasons why progress is slow in computerizing this process. In this paper, an expert system for preliminary aircraft design is studied with emphasis upon the design knowledge. The design knowledge is constructed by using the design-object knowledge and the designer's knowledge as the object-oriented representation. The design-object knowledge consists of design variables and relationships among design variables. The designer's knowledge consists of the designer's strategy for aircraft design. This system enables one to develop a flexible and dynamic design process according to a design condition.