Electrophoretic image displays and electrical twisting ball displays are the most promising candidates for Digital Paper. In the case of electrophoretic displays, their display elements consist of microcapsules that contain either negatively charged black or positively charged white particles (opposite pair of polarities are certainly allowed). Depending on the electric field applied, the particles migrate toward the top or bottom of the display. Electrical twisting ball displays consist of tiny balls whose surfaces are divided into two areas with different electric charges and colors (usually black and white). The electric field determines which side of each ball rotates upward. In both cases, the patterns of black and white can create letters, words and images.The key technology of these two systems is the difference in electric charge density of the display elements since this forms the force driving display element migration/rotation. Therefore, quantification of electric charge density is considered to be essential for controlling these systems. We define electrophoretic mobility as the parameter of electric charge density and study how the electric charge density influences the motion of the display elements using this parameter.We will also report a new method for preparing balls in twisting ball displays.