A method for adjusting an image on the basis of characteristics of a display system is provided. The image includes M horizontal lines. Each of the M horizontal lines respectively includes N pixels. Each pixel has an original gray level. A look-up table previously stores a plurality of conversion coefficients related to the characteristics of the display system. The method first calculates an ith loading according to the N original gray levels of the N pixels in the ith horizontal line. Based on the ith loading, an ith conversion coefficient corresponding to the ith loading is selected from the plurality of conversion coefficients in the look-up table. The method respectively multiplies the N original gray levels of the N pixels in the ith horizontal line by the ith conversion coefficient to generate N new gray levels for the N pixels in the ith horizontal line, whereby the image is adjusted.