Abstract: The developmental characteristics of microfractures in No.3 and No.15 coal seams in southern Qinshui basin were studied by means of scanning electron microscope, fluorescence microscope, X ray diffraction and so on. The relationship between microfracture and influencing factors including crustal stress, micropetrological unit, metamorphic grade and mineral were summarized. The main factors affecting the development of microfractures in coal reservoir were discussed. The results show that the microfractures in coal reservoir of this area are relatively developed and the density is generally 7~59 strips per 9 cm². Microfractures of type D and type C dominate. Tensile fractures and shear fractures that make relatively larger contribution to the permeability are common in scanning electron microscope, whereas endogenetic fractures are rare. Microfractures are generally developed in vitrinite component. Microfracture density decreases with the increase of metamorphic grade. The mineral filling effect in coal has great influence on the development of microfracture and destroys the permeability of coal directly. The magnitude and direction of crustal stress field control the opening degree of exogenous microfracture and then affect coal permeability. Therefore, crustal stress and mineral in coal are the main factors that influence the development of microfractures in coal reservoir of this area.