Mobile handset technology has improved greatly in recent decades. Improvements in hardware and software conduce the importance of the handset as a computer. Whilst the mobile phone industry and researchers emphasise understanding users from sociological aspects, such as services, usage habits, functionalities and so forth; this research aims to seek the variables that impact upon users’ interactions with the phone from the root-individual differences. For the purpose of placing mobile phone users at the core of the interface design process, the individual differences of gender, cognitive style and user experience were applied as the main variables in this research. Accordingly, to examine individuals’ differences and their operational behaviour with phone interfaces, the effects of cognitive style and user experience present stronger impact on the user performance than gender. The two factors were then investigated with regard to individuals’ operational behaviour with phone interfaces. This research then developed categories to link the theoretical attributes of cognitive style and phone users’ practical operational behaviour. Furthermore, the impact of cognitive style on the users’ operational behaviour was also connected to individuals’ problem solving ability. Meanwhile, the effect of user experience was investigated with a novel methodology-Taxonomy of Experience. The results revealed necessary elements that the users cared about the most, and these differed from the interface design guidelines of phone industry. This PhD thesis presents the application of the triangulation method to explore the variables that impact upon individuals’ interaction with mobile phone interfaces, specially concerned with mobile phone interface design from a psychological perspective, and provides a deeper understanding of users for future design events. More importantly, this research delivered the concept of understanding users before conducting design.