Multilevel Flow Model (MFM) is originated from Abstraction Hierarchy to describe a system process in means-end and part-whole way to make the process understandable to operators or other people. Research on MFM has been continued for about 20 years on measurement validation, alarm analysis, fault diagnosis and operation prediction, etc. Those methods based on MFM have been applied on various man-made machines and proved that the MFM is a very effective tool for reasoning about plant failure and making control strategies. However, the concrete usability evaluation of this modeling method has seldom been researched especially on the aspect of helping people understand the plant process and made reasoning based on the model. This paper conducts an evaluation experiment by tasks test based on MFM of a PWR plant mainly in two aspects, “understand the internal process of plant process based on MFM” and “make causal reasoning based on MFM”. With the assistance of an eye tracking equipment, the usability of the MFM is evaluated by subjects’ performance and their eye movement – which can reflect their cognitive work in the task test. Some preliminary results are concluded in this paper.