Abstract: Fractal dimension, as the characteristic parameter to describe surface topography, relates closely to the grey level threshold θ in the binarization of grey image. In the binarization process, the rationally selected binarization threshold θ* can characterize well the surface topography by way of extracting the fractal dimensions from the objects to investigate. It can be seen from some instances that there are the maximum and minimum thresholds θmax and θmin. When θ > θmax or θ < θmin, all the information obtained from binarization to calculate fractal dimensions will be in saturated state. With θmax and θmin taken into account, the empirical criterion θz.ast; can be got as the rationally selected one.