Abstract: Coal seam is typical double porosity medium, whose permeability is controlled by the structural parameters of pores and fractures.A unified mathematical model of porosity and permeability for gas flow in a double porosity medium was established.Lots of gas seepage phenomena in coal body were divided into three types:pore control, fracture control and pore-fracture combination control.With the gas seepage experimental data of six groups of coal samples and their test statistics of the porosity and fracture, the reasons of the permeability difference in the coal samples were discussed.The study found that the structural parameters of the pores and fractures determined the compression coefficient and permeability index of coal body, and then determined the type of gas seepage.The key factors affecting the permeability of coal seam were the density and the scale of pores and fractures.However, the stress sensitivity was almost impossible to be reflected in the micro pores because of the Van der Waals force between gas molecules and other particles.