A display device, comprising: a display panel comprising pixels and adapted to display a still image and a motion picture; a signal controller adapted to control signals for driving the display panel; and a graphics processing unit adapted to transmit input image data, a still image start signal and a still image end signal to the signal controller, a light source unit adapted to irradiate light to the display panel; and a light source driver adapted to generate a PWM signal to drive the light source unit, wherein the signal controller includes a frame memory adapted to store the input image data, andwherein the signal controller is adapted to drive the display panel at a first frequency when the still image end signal is applied for displaying the motion picture and at a second frequency that is lower than the first frequency when the still image start signal is applied for displaying the still image characterized in that the display panel is a normally black mode, wherein when the display panel is driven from the first frequency to the second frequency, the charging time of each pixel increases causing the fully charged amount in said each pixel to increase and thus its luminance, and the signal controller is adapted to control the light source driver such that the light source driver drives the light source unit at a first duty ratio when the display panel is driven at the first frequency, and drives the light source unit at a second duty ratio when the display panel is driven at the second frequency, wherein the second duty ratio is lower than the first duty ratio to compensate the increased luminance. a display panel comprising pixels and adapted to display a still image and a motion picture; a signal controller adapted to control signals for driving the display panel; and a graphics processing unit adapted to transmit input image data, a still image start signal and a still image end signal to the signal controller, a light source unit adapted to irradiate light to the display panel; and a light source driver adapted to generate a PWM signal to drive the light source unit, wherein the signal controller includes a frame memory adapted to store the input image data, andwherein the signal controller is adapted to drive the display panel at a first frequency when the still image end signal is applied for displaying the motion picture and at a second frequency that is lower than the first frequency when the still image start signal is applied for displaying the still image characterized in that the display panel is a normally black mode, wherein when the display panel is driven from the first frequency to the second frequency, the charging time of each pixel increases causing the fully charged amount in said each pixel to increase and thus its luminance, and the signal controller is adapted to control the light source driver such that the light source driver drives the light source unit at a first duty ratio when the display panel is driven at the first frequency, and drives the light source unit at a second duty ratio when the display panel is driven at the second frequency, wherein the second duty ratio is lower than the first duty ratio to compensate the increased luminance.