A display device for an air conditioning system of a vehicle may include a display unit of displaying brightness variations on a screen of the display for a predetermined time period, and a control unit of controlling the display to vary brightness on the screen of the display from a high intensity to a low intensity and vice versa while varying the length of time taken to vary the brightness. The control unit controls the display to perform a welcome function that gradually decreases the length of time when electric power may be applied to the display, and a goodbye function that gradually increases the length of time when an off button may be pushed, thereby obtaining an effect of making a dynamic impression or a static impression to meet a recent sensible trend.