This paper proposes an approach to quantify the information flow of diagnosis tasks in nuclear power plants. This paper presents a method to quantify cognitive information flows in diagnosis tasks, integrating a stage model (a qualitative approach) with information theory (a quantitative approach). The focus of the method is on (i) developing a stage model to describe information flow for diagnosis tasks, and (ii) quantifying the information flow. We develop an information flow model which consists of four stages: perception, identification, diagnosis, and planning. According to the knowledge it contains, five categories of the state of information are defined. Then, the information flow of the model is quantified using Conant's model. Finally, we apply this method to seven diagnosis tasks described in diagnosis procedure of the emergency operating procedure for reference plant A (combustion engineering type) and investigate the applicability of this approach. (C) 2003 Elsevier Science Ltd. All rights reserved.