An apparatus for testing a display device includes a display device to display test patterns, a graphic process unit to supply analog mode signals and digital mode signals to the display device, and a control unit to allow test patterns of an analog testing mode and test patterns of a digital testing mode to be sequentially displayed on the display device upon receiving a control signal from the graphic process unit, and to output a control signal to the display device to sequentially change display characteristics of an image according to an on-screen-display mode testing menus.