A method of controlling a display device including a touch screen is provided. The method includes displaying, on the touch screen, a plurality of windows respectively corresponding to executing applications, such that the windows do not overlap each other, displaying a plurality of boundary lines differentiating the plurality of windows, and a center button disposed at an intersection point of the plurality of boundary lines, receiving, through the touch screen, an input of a window size change command to change a size of at least one of the plurality of windows, and resizing and displaying at least one of the plurality of windows in correspondence to the window size change command.