Abstract: The pore structure of coal is complex and it is difficult to characterize pore structure by traditional Euclidean geometry. However, fractal theory is an effective method to investigate the irregularities of pore structure. The fractal dimensions of seepage-pores of 10 coal samples collected from Hancheng mining area in Weibei coalfield were obtained using mercury injection experiment based upon thermodynamic fractal model and the geometric properties of pore were quantitatively characterized. The relationship between fractal dimension and permeability was also discussed. The results show that the more adsorption-pores, the higher fractal dimension for the same coal rank. The fractal dimension has a negative correlation with the petrologic permeability of coal. The fractal dimension of seepage-pores of coals can be used as one of the quantitative indexes for evaluation of permeability of coal reservoir.