Abstract: The functional requirements and design of mobile phone screen magnifier is discussed in this paper. Investigation is made with older and those whose vision capability is bad as subjects. With a series of interviews in groups, the users daily life experience and their requirements are acquired. Then, a set of critical design principles for mobile screen magnifier is developed. Based on the principles, two conceptual models are designed to address users requirements and design principles. In order to test and verify those principles, various types of prototypes (paper prototype, flash demo, and video demo) are developed and evaluated by a group of experts in the fields of human computer interaction and usability testing. Consequently, both models are recognized by the experts and seven of the design principles are selected as design guide.