In this paper, a new method of product function analysis is presented to categorize various design information generated in product development processes. In order to improve product functions, designers must understand unit functions and modified parts of products. The product function analysis (PFA) is based on the designer's understanding of product functions. The proposed PFA provides the methodology to classify the various functions systematically and understand the relation between functions easily. Using this approach, efficient design support system can be developed and used for designers to support decision-making with design knowledge.