Abstract: In order to find out the fragmentation of coal in the miming process affected to the initial velocity of gas emission, from the view of the study on the diffici ulty degree of the coal and gas outburst, the paper had a study on the particle size variation of the different metamorphic degree coal sample affected to the initial veloc ity of gas emission.The results showed that the initial velocity of the emission from the coal sample would be decreased in a power function with the particle size increa sed.The higher fragmentation of the coal was, the higher initial velocity of gas emission would be and the increasing tendency would be getting lower with the particle si ze increased.The higher metamorphic degree of the coal sample, the particle size of the coal sample would have a higher influence to the initial velocity of the gas emis sion.As for the coal sample with low Protodyakonov coefficient, the initial velocity of gas emission would be highly affected by the particle size variation of the coal sam ple.