This paper presents the proposal for the design of a measurement of cognitive process of nurses at acute care setting when they make cognitive error. nurses especially at acute care setting perform their numerous nursing tasks simultaneously in time-stressed, information-rich situation. To cope with these severe cognitive demands expertise nurses develop simplification strategies not to consume their mental resources so much. However medical errors happen, regardless of nurses' experience. The resource allocation theory of Norman and Bobrow suggests that inappropriate selection of simplification strategy when the task is needed more attention induce the cognitive error. In this paper too much mental workload is presumed one of the reasons to make inappropriate selection. And the study design to measure cognitive process under the different mental workload situation by talk aloud protocol and psycho-physiological response is proposed.