A method for adjusting a target gray level of a target pixel based on a spatial mask is provided. The target pixel is surrounded with N adjacent pixels; each of the N adjacent pixels respectively has an adjacent gray level. The method first multiplies the target gray level with a sharpness index to generate an amplified target gray level and respectively multiplies each of the adjacent gray level with an adjacent index to generate N modified adjacent gray levels. Then, the amplified target gray level and the N modified adjacent gray levels are added up to generate a first gray level sum. The method multiplies the first gray level sum with a multiplying parameter to generate a second gray level sum. At last, the method divides the second gray level sum by a dividing parameter to generate an adjusted gray level for the target pixel.