This study attempted to discuss the design principles for enhancing usability in terms of asymmetry of mental models between users and designers. If the user model is asymmetrical to the design model, i.e., the degree of agreement between models is low and the user's mental model is not compatible with the actual system, the user cannot operate the system properly, which may cause errors. The aim of this study was to investigate the characteristics of users who have asymmetrical mental models and identify what such users did not understand. Twenty-four subjects participated in an experiment that involved a digital camera operation task and mental model tests that measure the level of the agreement of the mental model construction in terms of functional and structural models. Initially, the participants were grouped based on mental model test scores: symmetrical mental model group (n = 17) and asymmetrical mental model group (n = 7). Then, the groups were compared in terms of performance and subjective comprehension. The comparison indicated that the symmetrical mental model group performed more quickly and accurately than the asymmetrical group. The results also confirmed that the asymmetrical mental model group had a lower level of comprehension in terms of understanding the device status, detecting and responding to device status changes, and understanding the hierarchical structure of the screen
