Abstract: Computerized displays are the basic elements of the human-machine interface in the digital nuclear power plant, of which the complexity will impact the situation awareness of operators. In order to make a fast and effective quantitative assessment of complexity of a display, a method based on the image entropy was presented. The human machine interface was converted into a gray scale image, and then the entropy of the display was calculated using the image entropy formula as a measurement of the complexity. The experiment result shows that the complexity measurement of the display based on the image entropy is according with the assessment result by human being, and can reflect the cognitive feeling of the operator. This method can be used in practice, especially at the early stage of the design, where the validation by experiment or by expert is not ready. It can be used to measure the complexity of a display to avoid a too complex design and to improve the human-machine interaction.