By employing a mercury injection porosimetry, the evolution of micro-pores in marbles is measured as a function of the stresses experienced. Based on the fractal model for porous media, the fractal dimensions of micro-pores under different stress levels are estimated. The study shows that porous rocks are actually fractal structures, the fractal dimension of the pore volume increases with the increase of load. This suggests that the fractai dimension of the pore volume could be used as an indicator of damage in the material, which provides a new way of defining the damage variable.