A display device includes a display panel which displays an image in a normal mode or low power mode. A DC-DC converter supplies a source driving voltage to the data driver, and supplies a first power voltage to the power line of the display panel in the normal mode. The data driver supplies a first auxiliary power voltage to the power line of the display panel in the low power mode. The DC-DC converter gradually changes the voltage level of the first power voltage to a reference output voltage level over a first period when the display panel initially enters into the normal mode, and changes the voltage level of the first power voltage to the reference output voltage level over a second period shorter than the first period when the mode of the display panel is switched from the low power mode to the normal mode.