KHNP (Korea Hydro & Nuclear Power Co.) defines and manages equipment of Nuclear Power Plants systematically with functional importance determination of each equipment for efficient maintenance and optimal preventive maintenance. But the existing functional importance determinations have some different results between the plants, systems and engineers due to gap of understanding of classification criteria because they have been done in terms of equipment level rather than function level. so that caused the repeated work. To make up for this problem improve methodology of functional importance determination using MR (Maintenance Rule) and do classification of equipment for new nuclear power plants based on function level. In addition, methodical documentation for basis of importance determination is done to help that system engineers can easily understand and use.