Abstract: In order to discuss the low permeability cause of No.15 seam in Shizhuang Area at the south part of Qinshui Basin,based on the drling and mine samples from two major seams of No.15 seam and No.3 seam in Shizhuang Area as the study objects ,a cryogenic liquid nitrogen.,pressurized mercury test,scanning electron microscope,microscopy.X-ray difraction,total sulfur and morpholgical sulfur analysis as well as othermethods were applied to the comparison study on the pore structure features to control the permeability of No.15 seam,the pore-crack development features and mineral development features in coal and other seam factors as well as the sedimentary environment,buried depth and other seam formation conditions. The results showed that the pore structure of No.15 seam was poor and the porecrack was seriously backiled.The mineral development in coal high ash content,high vertical stress and other factors were the causes to make the low permeability of No.15 seam.