An OLED display device comprising: a display panel comprising a gate line, a data line, and a pixel at a crossing region of the gate line and the data line; a timing controller configured to generate a gate driving control signal, a data driving control signal, and a power control signal based on a display period corresponding to a time interval of frames; a gate driver configured to provide a gate signal to the pixel through the gate line based on the gate driving control signal; a data driver configured to provide a data signal to the pixel through the data line based on the data driving control signal; and a power supply configured to generate a power voltage to drive the pixel, wherein a driving transistor is configured to drive the pixel by providing a current through a light emission element during a light emission period; wherein the timing controller is configured to: select one of a plurality of display periods of different lengths as the display period based on input image data; determine whether an input image corresponds to a video, or corresponds to a still image, based on the input image data; select a first display period among a plurality of display periods when the input image corresponds to the video; and select a second display period among the plurality of display periods, which is greater than the first display period, when the input image corresponds to the still image, wherein when the second display period is selected, the timing controller is configured to generate the power control signal based on a luminance profile that comprises information of luminance change over time during the display period caused by the variation of threshold voltage of the driving transistors under a constant DC current, and wherein the power supply is configured to vary the power voltage to, either gradually or step-by-step, increase the power voltage across the pixels based on the power control signal during the second display period. a display panel comprising a gate line, a data line, and a pixel at a crossing region of the gate line and the data line; a timing controller configured to generate a gate driving control signal, a data driving control signal, and a power control signal based on a display period corresponding to a time interval of frames; a gate driver configured to provide a gate signal to the pixel through the gate line based on the gate driving control signal; a data driver configured to provide a data signal to the pixel through the data line based on the data driving control signal; and a power supply configured to generate a power voltage to drive the pixel, wherein a driving transistor is configured to drive the pixel by providing a current through a light emission element during a light emission period; wherein the timing controller is configured to: select one of a plurality of display periods of different lengths as the display period based on input image data; determine whether an input image corresponds to a video, or corresponds to a still image, based on the input image data; select a first display period among a plurality of display periods when the input image corresponds to the video; and select a second display period among the plurality of display periods, which is greater than the first display period, when the input image corresponds to the still image, select one of a plurality of display periods of different lengths as the display period based on input image data; determine whether an input image corresponds to a video, or corresponds to a still image, based on the input image data; select a first display period among a plurality of display periods when the input image corresponds to the video; and select a second display period among the plurality of display periods, which is greater than the first display period, when the input image corresponds to the still image,