According to the data of 146 coal samples measured by mercury penetration, coal pores are classified into two levels of ＜65 nm diffusion pore and ＞65 nm seeping pore by fractal method based on the characteristics of diffusion, seepage of coal bed methane(CBM) and on the research results of specific pore volume and pore structure. The diffusion pores are further divided into three categories: ＜8 nm micropore, 8-20 nm transitional pore, and 20-65 nm minipore based on the relationship between increment of specific surface area and diameter of pores, while seepage pores are further divided into three categories: 65-325 nm mesopore,325-1000 nm transitional pore, and ＞1000 nm macropore based on the abrupt change in the increment of specific pore volume.