A display apparatus comprising: a display unit which comprises a plurality of pixels having an organic light emitting diode (OLED); a power supply arranged to supply power to the display unit; an image processor comprising a brightness compensating unit arranged to process an input image to display an image on the display unit, wherein the brightness compensating unit comprises an APL extractor arranged to extract an APL of the input image, a histogram calculator, a maximum gray scale extractor and a compensation processor; and a controller arranged to determine whether an average picture level, APL, of the input image is greater than or equal to a predetermined value, and, if the extracted APL value is greater than or equal to the predetermined value, to control the histogram calculator to calculate a histogram of the input image; to control the maximum gray scale extractor to extract a maximum gray scale of the calculated histogram; and to control the compensation processor to compensate for the gray scales of the input image based on a compensation coefficient that is set so that the extracted maximum gray scale becomes equal to a predetermined gray scale value after compensation for the gray scales to thereby increase a brightness of the input image, and to control the power supply to reduce the power supplied to the display unit in correspondence with the increase of brightness caused by the compensated gray scales such that a brightness of an image displayed on the display unit after compensating the gray scales is the same as the brightness of the input image before compensating the gray scales, and, if the extracted APL value is less than the predetermined value, to output the input image to the display unit without compensating the gray scales. a display unit which comprises a plurality of pixels having an organic light emitting diode (OLED); a power supply arranged to supply power to the display unit; an image processor comprising a brightness compensating unit arranged to process an input image to display an image on the display unit, wherein the brightness compensating unit comprises an APL extractor arranged to extract an APL of the input image, a histogram calculator, a maximum gray scale extractor and a compensation processor; and a controller arranged to determine whether an average picture level, APL, of the input image is greater than or equal to a predetermined value, and, if the extracted APL value is greater than or equal to the predetermined value, to control the histogram calculator to calculate a histogram of the input image; to control the maximum gray scale extractor to extract a maximum gray scale of the calculated histogram; and to control the compensation processor to compensate for the gray scales of the input image based on a compensation coefficient that is set so that the extracted maximum gray scale becomes equal to a predetermined gray scale value after compensation for the gray scales to thereby increase a brightness of the input image, and to control the power supply to reduce the power supplied to the display unit in correspondence with the increase of brightness caused by the compensated gray scales such that a brightness of an image displayed on the display unit after compensating the gray scales is the same as the brightness of the input image before compensating the gray scales, and, if the extracted APL value is less than the predetermined value, to output the input image to the display unit without compensating the gray scales. to control the histogram calculator to calculate a histogram of the input image; to control the maximum gray scale extractor to extract a maximum gray scale of the calculated histogram; and to control the compensation processor to compensate for the gray scales of the input image based on a compensation coefficient that is set so that the extracted maximum gray scale becomes equal to a predetermined gray scale value after compensation for the gray scales to thereby increase a brightness of the input image, and to control the power supply to reduce the power supplied to the display unit in correspondence with the increase of brightness caused by the compensated gray scales such that a brightness of an image displayed on the display unit after compensating the gray scales is the same as the brightness of the input image before compensating the gray scales, and, if the extracted APL value is less than the predetermined value, to output the input image to the display unit without compensating the gray scales.