The aim of present study was to improve the safety and the effectiveness of the in-vehicle display for camera monitor systems (CMS). The experimental factors were the size of the display and the location of the display. The participants were required to simultaneously carry out a tracking task and a rearward monitoring task. The evaluation measures were: the reaction time and the percentage correct reactions to the rearward stimuli, the tracking error, the car distance between own and the preceding car, and the horizontal neck movement. As a result, the reaction time, the percentage of correct reaction, the neck movement of CMS were better than the traditional side mirrors. It suggests CMS is safer and more effective than the side mirrors. Particularly, more than 5-in. display which was located at smaller visual angle had faster and more accurate reaction. This condition would be one of the design requirements for CMS.