A display device including: a display panel including a plurality of pixels configured to be driven in a normal mode or an always on display mode; an illuminance sensor configured to detect illuminance of ambient light; a direct current (DC)-DC converter configured to supply a power supply voltage having a first voltage level to the pixels through a power supply line when the illuminance is greater than a predetermined reference value in the always on display mode; and a display panel driver configured to supply a power supply voltage having a second voltage level to the pixels through the power supply line when the illuminance is less than or equal to the predetermined reference value in the always on display mode.