Abstract: In order to study the gas diffusion in coal underground,the gas desorbing process was recorded when coal particles within 0. 25-0. 5,0. 5-1 and 1-2 mm were applied to axial stresses of 0-12 MPa,respectively. Then,the ex- perimental data was fitted for the diffusion coefficient using the unipore model and the bidisperse model. According to the relationship between the diffusion coefficient and the porosity,the dynamic evolution model of the gas diffusion co- efficient was established,which is in accordance with the experimental results. Results show that the bidisperse model can match the experimental data more perfectly than the unipore model. The diffusion coefficients of macro-pores were on the order of 10-4 s-1 ,which were 1-2 order higher than that of micro-pores. Both the desorbed gas and diffusion co- efficients increased with the increasing stresses. The desorbed gas and diffusion coefficients increased with the decrea- sing size of coal particle. Coal samples with larger size were more sensitive to the stress,and the macro-pores showed more sensitive to stresses than the micro-pores. This study would provide some reference to understand gas diffusion in deformed coal underground.