Abstract: Permeability is a main parameter for evaluation of water conductivity of porous media, and it is a macroscopic feature of micro-pore structure of water-conducting media. To obtain the geometry parameters of microscopic pores, we performed scanning electron microscopy(SEM) on sandstone samples. The image processing techniques were also applied in calculation of different statistical parameters(e.g., fractal dimensions) and geometrical parameters. Then, fractal theory was introduced to investigate the influence of micro pore structure on permeability. At the same time, the minimum pore radius r_min interval acting on the seepage was obtained by the measured permeability values. This interval can be used as the optimal interval of the minimum pore radius r_min when we have a prediction for permeability. The corresponding percentage of pore accumulation interval was also determined. The results show that:the value of minimum pore radius r_min of sandstone has a great influence on permeability. With increase of the value of minimum pore radius, its permeability decreases sharply at first, then tends to be stable gradually, is finally accompanied by an upward trend. In compact sandstone, when theoretical permeability is equivalent to measured one, the minimum pore radius r_min falls within the interval of 15%-25% of the cumulative percentage of pore radius, and the corresponding pore radius interval can be used as the optimal interval of the minimum pore radius r_min when we calculate the theoretical permeability of sandstone. Meanwhile, with the increase of the permeability of sandstone, corresponding to the minimum pore radius, the cumulative percentage of pore radius increases.