This study speculated that the frustration of using mobile phone for elderly users results from not only the physical inconvenience of bodies but importantly from the discrepancy between user and designer mental models the system image of mobile phones. From the viewpoint of cognitive psychology, this paper first developed an appropriate research method for the elderly subjects to understand their thinking processes during mobile phone operation. The reason causing frustration and the deficiency of the current user interface were analyzed and discussed. Main findings indicated that current interfaces were poorly designed for elderly persons. Some of the subjects could not even understand the basic concepts of the interface of mobile phone, for example, hierarchy and soft key. Finally, suggestions for mobile phones designers were proposed.