A method of generating a gain of an image frame according to a look up table of gain which is set up based on luminance sensitivity of human eyes is proposed. The method includes setting a gain of an image frame to 1, scanning images of a plurality of front rows of the image frame, averaging the images of the plurality of the front rows of the image frame to generate an average value of the images of the plurality of the front rows of the image frame, finding a gain from the look up table of gain according to the average value of the images of the plurality of the front rows of the image frame, and adjusting remaining rows of the image frame according to the gain to generate images of the remaining rows of the image frame.