To understand the influence of gas pressure on the evolution of coal permeability, the permeability experiments of coal samples collected from 9# coal seam (high gas coal seam) in Tangshan Coal Mine of Hebei Province, China were performed under different gas (helium and nitrogen) pressure; deformations induced by nitrogen adsorption were obtained simultaneously. Besides the pore structure characteristics of coal samples were acquired by the mercury injection porosimetry (MIP) experiment, and pore surface fractal dimensions were obtained. As the nitrogen pressure increases from 0.3 MPa to 3 MPa, adsorption strain increases from 0.168×10^-3 to 1.076×10^-3, and the range of increase decreased gradually, but the permeability of coal samples decreases from 16.05×10^-18m² to 4.91×10^-18m² and then rises to 5.69×10^-18m² are demonstrated. The trend of permeability for helium is consistent with that for nitrogen, and the average permeability for helium is 1.42~1.88 times as much as that for nitrogen under the same gas pressure. The gas pressure corresponds to the lowest value of permeability for helium and nitrogen are 1.5 MPa and 2.5 MPa, respectively. Consequently, the absorptivity of gas is crucial to the permeability evolution of coal samples. According to the mercury intrusion porosimetry data, coal matrix compressibility is 7.2×10^-11m²/N based on, and the corrected porosity of coal samples is 53.8% considering coal matrix compression. Additionally, the seepage pores with pore sizes larger than 100 nm are relatively developed, accounting for 80.4% of the total pore volume, which is conducive to gas seepage. The surface fractal dimension D_s1 is larger than D_s2 and D_s3, and D_s1 is positively correlated with micropore volume content, while D_s2 and D_s3 are negatively correlated with pore volume content and gas permeability.
 

coal permeability; gas pressure; adsorption strain; pore content; coal matrix compressibility