While engineering a power plant layout, objective functions and constraints of the power plant frequently conflict each other in terms of factors such as safety, economic costs, maintainability, and construction term of works. This paper proposes a multi-objective layout optimization system for a power plant through effective interaction between the designer and the computer; this system is developed by applying an interactive Satisficing Trade-Off Method (STOM) for multiple-criteria decision-making. In order to obtain the layout candidates for decision-making, the authors present a suitable genetic algorithm (GA) approach to arrange the plot plan for the layout of buildings and equipment in a site. In this GA approach, the coding method and evolution operation method used for obtaining a diverse optimal layout solution are employed with a low computational load and simple design information. Finally, the validity and usefulness of this proposed system is demonstrated by solving a layout design problem for an actual power plant.