||It is now possible to record and collect a large amount of phone calls in a city. Each phone call record usually contains the caller and callee IDs, date and time, and the base station where the phone calls are made. As mobile phones are widely used in our daily life, many human behaviors can be revealed by analyzing mobile phone data. In this paper, we propose a comprehensive visual analysis system which can be used to analyze the population's mobility pattern from millions of phone call records. Our system consists of three major components: 1) visual analysis of user groups in a base station; 2) visual analysis of the mobility pattern of different user groups making phone calls in certain base stations; 3) visual analysis of handoff phone call records. Some well established visualization techniques such as parallel coordinates and pixel-based representations have been integrated into our system. We also develop some novel visualization schemes such as Voronoi-diagram-based visual encoding to reveal the unique features of mobile phone data. We have applied our system to real mobile phone data collected in a large city and some interesting findings regarding people's mobility pattern have been obtained. Keywords: Visual analysis, mobility pattern, mobile phone.