Abstract: Coal as a porous media, the properties of its pores will affect the enrichment and diffusion of coal bed gas directly so that it is very essential to discuss the pore characteristics of coal.By using the data from the mercury-injection tests and low temperature nitrogen adsorption tests, the authors analyzed the pore shape, types and distribution of the Upper Permian coal samples in western Guizhou systematically.Moreover, the geological control of the pore occurrence in the coal reservoir was discussed in the light of the coal maceral and rank.The results indicate that the porosity of the coal reservoir ranges from 1.4% to 9.5%, and decreases from the north to the south.Most types of pores are adsorption pores.In addition, semi-enclosed pores are the major ones in the high-rank coal.Comparing with the low-rank coal, the permeability and connectivity of the high-rank coal are relatively poor, but better than those in Qinshui basin generally.The coal rank is the main factor which affects the occurrence of the porosity in the coal seams.That is to say, the porosity shows the high-low-high tendency with increase of coal rank.