Abstract: In order to analyze influence of coal particle size on initial speed of methane diffusion, the relationship of particle size and initial speed of methane diffusion of coal samples with different moisture content was tested by use of WT-1 type initial speed of methane diffusion tester. The coal samples were collected in No.4 coal seam in a coal mine with different particle sizes of less than 7 mesh, 7-10 mesh, 10-18 mesh, 18-35 mesh and different moisture content of natural drying, drying, 3%, 5% and 7%. And the initial speed of methane diffusion of coal samples of natural drying and drying for 5 h were tested, which were with different particle sizes of less than 7 mesh & 35-60 mesh,7-10 mesh & 35~60 mesh, and 10-18 mesh & 35-60 mesh under different proportion of 6∶1,5∶2,4∶3,3∶4,2∶5 and 1∶6. The experimental results show that initial speed of methane diffusion of coal samples with different moisture content increases as the particle size decreases, and the particle size smaller, the increasing trend is more obvious; the proportion of coal particles with small particle size in coal seam larger, the initial speed of methane diffusion of coal samples is larger.