A method for adjusting image contrast has the steps of constructing a luminance histogram of an original image; acquiring an image characteristic of the luminance histogram; judging whether there is a peak wave or deviation of luminance existed in that image characteristic; determining a control point after performing a specific algorithm; deriving a linear equation from the control point and a contrast adjusting value in a Cartesian coordinate system; establishing a gamma table from the correspondent relationship between the horizontal and vertical axis in the coordinate, where the relationship can be calculated according to the linear equation; and finally generating a luminance value from the gamma look-up table using the input luminance of each pixel in the original image.